=encoding utf-8

=head1 NAME

Apocalypse_05 - Pattern Matching

=head1 AUTHOR

Larry Wall <larry@wall.org>

=head1 VERSION

  Maintainer: Larry Wall <larry@wall.org>
  Date: 4 Jun 2002
  Last Modified: 9 Dec 2004
  Number: 5
  Version: 5

This is the Apocalypse on Pattern Matching, generally having to do with
what we call "regular expressions", which are only marginally related
to real regular expressions. Nevertheless, the term has grown with the
capabilities of our pattern matching engines, so I'm not going to try
to fight linguistic necessity here. I will, however, generally call
them "regexes" (or "regexen", when I'm in an Anglo-Saxon mood).

Here are the RFCs covered in this Apocalypse. PSA stands for "problem,
solution, acceptance", my private rating of how this RFC will fit into
Perl 6. Doubtless I have misclassified I<your> RFC, though the other
ratings are pretty accurate. C<:-)>

    RFC   PSA   Title
    ---   ---   -----
    072   aaa   Variable-length lookbehind. 
    093   abb   Regex: Support for incremental pattern matching
    110   bbb   counting matches
    112   acc   Assignment within a regex
    135   acr   Require explicit m on matches, even with ?? and // as delimiters.
    144   aaa   Behavior of empty regex should be simple
    145   acr   Brace-matching for Perl Regular Expressions
    150   acc   Extend regex syntax to provide for return of a hash of matched subpatterns
    156   aaa   Replace first match function (C<?...?>) with a flag to the match command.
    164   ccr   Replace =~, !~, m//, s///, and tr// with match(), subst(), and trade()
    165   acc   Allow Variables in tr///
    166   abc   Alternative lists and quoting of things
    191   bbc   smart container slicing
    197   cdr   Numeric Value Ranges In Regular Expressions
    198   adr   Boolean Regexes
    261   dbr   Pattern matching on perl values
    274   acc   Generalised Additions to Regexs
    276   aaa   Localising Paren Counts in qr()s.
    308   dar   Ban Perl hooks into regexes
    316   bcr   Regex modifier for support of chunk processing and prefix matching
    317   aaa   Access to optimisation information for regular expressions
    331   acc   Consolidate the $1 and \1 notations
    332   abc   Regex: Make /$/ equivalent to /\z/ under the '/s' modifier
    348   bcc   Regex assertions in plain Perl code
    360   acb   Allow multiply matched groups in regexes to return a listref of all matches
    361   abb   Simplifying split()

Interestingly, there were no withdrawn RFCs for pattern matching. That
means either that there were no cork-brained ideas proposed, or that
regex culture is so cork-brained already that the cork-brained ideas
blend right in. I know where my money is... C<:-)>

In fact, regular expression culture is a mess, and I share some of the
blame for making it that way. Since my mother always told me to clean
up my own messes, I suppose I'll have to do just that.

For prior Apocalypses, I've used the RFCs as a springboard for
discussion of my thinking, but this one is special, because none of the
RFCs were courageous enough (or foolhardy enough) to look at the big
picture and propose radical change where it's needed. But Perl has
often been tagged as a language in which it's easy to write programs
that are difficult to read, and it's no secret that regular expression
syntax that has been the chief culprit. Funny that other languages have
been borrowing Perl's regular expressions as fast as they can...

That's primarily because we took several large steps in Perl 5 to
enhance regex capabilities. We took one large step forwards with the
C</x> option, which allowed whitespace between regex tokens. But we
also took several large steps sideways with the C<(?...)> extension
syntax. I call them steps sideways, but they were simultaneously steps
forward in terms of functionality and steps backwards in terms of
readability. At the time, I rationalized it all in the name of backward
compatibility, and perhaps that approach was correct for that time and
place. It's not correct now, since the Perl 6 approach is to break
everything that needs breaking all at once.

And unfortunately, there's a lot of regex culture that needs breaking.

Regex culture has gone wrong in a variety of ways, but it's not my
intent to assign blame--there's plenty of blame to go around, and
plenty of things that have gone wrong that are nobody's fault in
particular. For example, it's nobody's fault that you can't
realistically complement a character set anymore. It's just an accident
of the way Unicode defines combining characters. The whole notion of
character classes is mutating, and that will have some bearing on the
future of regular expression syntax.

Given all this, I need to warn you that this Apocalypse is going to be
somewhat radical. We'll be proposing changes to certain "sacred"
features of regex culture, and this is guaranteed to result in future
shock for some of our more conservative citizens. Do not be alarmed. We
will provide ways for you to continue programming in old-fashioned
regular expressions if you desire. But I hope that once you've thought
about it a little and worked through some examples, you'll like most of
the changes we're proposing here.

So although the RFCs did contribute greatly to my thinking for this
Apocalypse, I'm going to present my own vision first for where regex
culture should go, and then analyze the RFCs with respect to that
vision.

First, let me enumerate some of the things that are wrong with current
regex culture.

=over

=item * Too much history

=item * Too compact and "cute"

=item * Poor Huffman coding

=item * Too much reliance on too few metacharacters

=item * Different things look too similar

=item * Poor end-weight design

=item * Too much reliance on modifiers

=item * Too many special rules and boobytraps

=item * Backreferences not useful enough

=item * Too hard to match a literal string

=item * Two-level interpretation is problematic

=item * Too little abstraction

=item * Little support for named captures

=item * Difficult to use nested patterns

=item * Little support for grammars

=item * Inability to define variants

=item * Poor integration with "real" language

=item * Missing backtracking controls

=item * Difficult to define assertions

=back

I'm sure there are other problems, but that'll do for starters. Let's
look at each of these in more detail.

=head2 Too much history

Most of the other problems stem from trying to deal with a rich
history. Now there's nothing wrong with history per se, but those of us
who are doomed to repeat it find that many parts of history are
suboptimal and contradictory. Perl has always tried to err on the side
of incorporating as much history as possible, and sometimes Perl has
succeeded in that endeavor.

Cultural continuity has much to be said for it, but what can you do
when the culture you're trying to be continuous with is itself
discontinuous? As it says in Ecclesiastes, there's a time to build up,
and a time to tear down. The first five versions of Perl mostly built
up without tearing down, so now we're trying to redress that omission.

=head2 Too compact and "cute"

Regular expressions were invented by computational linguists who love
to write examples like C</aa*b*(cd)*ee/>. While these are conducive to
reasoning about pattern matching in the abstract, they aren't so good
for pattern matching in the concrete. In real life, most atoms are
longer than "C<a>" or "C<b>". In real life, tokens are more
recognizable if they are separated by whitespace. In the abstract,
C</a+/> is reducible to C</aa*/>. In real life, nobody wants to repeat
a 15 character token merely to satisfy somebody's idea of theoretical
purity. So we have shortcuts like the C<+> quantifier to say "one or
more".

Now, you may rightly point out that C<+> is something we already have,
and we already introduced C</x> to allow whitespace, so why is this
bullet point here? Well, there's a lot of inertia in culture, and the
problem with C</x> is that it's not the default, so people don't think
to turn it on when it would probably do a lot of good. The culture is
biased in the wrong direction. Whitespace around tokens should be the
norm, not the exception. It should be acceptable to use whitespace to
separate tokens that could be confused. It should not be considered
acceptable to define new constructs that contain a plethora of
punctuation, but we've become accustomed to constructs like
C<< (?<=...) >> and C<(??{...})> and C<[\r\n\ck\p{Zl}\p{Zp}]>, so we
don't complain. We're frogs who are getting boiled in a pot full of
single-character morphemes, and we don't notice.

=head2 Poor Huffman coding

Huffman invented a method of data compaction in which common characters
are represented by a small number of bits, and rarer characters are
represented by more bits. The principle is more general, however, and
language designers would do well to pay attention to the "other" Perl
slogan: Easy things should be easy, and hard things should be possible.
However, we haven't always taken our own advice. Consider those two
regex constructs we just saw:

    (?<=...)
    (??{...})

Which one do you think is likely to be the most common in everyday use?
Guess which one is longer...

There are many examples of poor Huffman coding in current regexes.
Consider these:

    (...)
    (?:...)

Is it really the case that grouping is rarer than capturing? And by two
gobbledygooky character's worth? Likewise there are many constructs
that are the same length that shouldn't be:

    (?:...)
    (?#...)

Grouping is much more important than the ability to embed a comment.
Yet they're the same length currently.

=head2 Too much reliance on too few metacharacters

A lot of our Huffman troubles came about because we were trying to
shoehorn new capabilities into an old syntax without breaking anything.
The C<(?...)> construct succeeded at that goal, but it was new wine in
old wineskins, as they say. More successful was the C<*?> minimal
matching hack, but it's still symptomatic of the problem that we only
had three characters to choose from that would have worked at that
point in the grammar. We've pretty nearly exhausted the available
backslash sequences.

The waterbed theory of linguistic complexity says that if you push down
one place, it goes up somewhere else. If you arbitrarily limit yourself
to too few metacharacters, the complexity comes out somewhere else. So
it seems obvious to me that the way out of this mess is to grab a few
more metacharacters. And the metacharacters I want to grab are...well,
we'll see in a moment.

=head2 Different things look too similar

Consider these constructs:

    (??{...})
    (?{...})
    (?#...)
    (?:...)
    (?i:...)
    (?=...)
    (?!...)
    (?<=...)
    (?<!...)
    (?>...)
    (?(...)...|...)

These all look quite similar, but some of them do radically different
things. In particular, the C<< (?<...) >> does not mean the opposite of
the C<< (?>...) >>. The underlying visual problem is the overuse of
parentheses, as in Lisp. Programs are more readable if different things
look different.

=head2 Poor end-weight design

In linguistics, the notion of end-weight is the idea that people tend
to prefer sentences where the short things come first and the long
things come last. That minimizes the amount of stuff you have to
remember while you're reading or listening. Perl violates this with
regex modifiers. It's okay when you say something short like this:

    s/foo/bar/g

But when you say something like we find in RFC 360:

    while ($text =~ /name:\s*(.*?)\n\s*
                    children:\s*(?:(?@\S+)[, ]*)*\n\s*
                    favorite\ colors:\s*(?:(?@\S+)[, ]*)*\n/sigx) {...}

it's not until you read the C</sigx> at the end that you know how to
read the regex. This actually causes problems for the Perl 5 parser,
which has to defer parsing the regular expression till it sees the
C</x>, because that changes how whitespace and comments work.

=head2 Too much reliance on modifiers

The C</s> modifier in the previous example changes the meaning of the
C<.> metacharacter. We could, in fact, do away with the C</s> modifier
entirely if we only had two different representations for "any
character", one of which matched a newline, and one which didn't. A
similar argument applies to the C</m> modifier. The whole notion of
something outside the regex changing the meaning of the regex is just a
bit bogus, not because we're afraid of context sensitivity, but because
we need to have better control within the regex of what we mean, and in
this case the context supplied outside the regex is not precise enough.
(Perl 5 has a way to control the inner contexts, but it uses the
self-obfuscating C<(?...)> notation.)

Modifiers that control how the regex is used as a whole do make some
sense outside the regex. But they still have the end-weight problem.

=head2 Too many special rules and boobytraps

Without knowing the context, you cannot know what the pattern C<//>
will do. It might match a null string, or it might match the previously
successful match.

The C<local> operator behaves differently inside regular expressions
than it does outside.

It's too easy to write a null pattern accidentally. For instance, the
following will never match anything but the null string:

    /
    | foo
    | bar
    | baz
    /x

Even when it's intentional, it may not look intentional:

    (a|b|c|)

That's hard to read because it's difficult to make the absence of
something visible.

It's too easy to confuse the multiple meanings of dot. Or the multiple
meanings of C<^>, and C<$>. And the opposite of C<\A> is frequently not
C<\Z>, but C<\z>. Tell me again, when do I say C<\1>, and when do I say
C<$1>? Why are they different?

=head2 Backreferences not useful enough

Speaking of C<\1>, backreferences have a number of shortcomings. The
first is actually getting ahold of the right backreference. Since
captures are numbered from the beginning, you have to count, and you
can easily count wrong. For many purposes it would be better if you
could ask for the last capture, or the one before that. Or perhaps if
there were a way to restart the numbering part way through...

Another major problem with backreferences is that you can't easily
modify one to search for a variant. Suppose you match an opening
parenthesis, bracket, or curly. You'll like to search for everything up
to the corresponding closing parenthesis, bracket, or curly, but
there's no way to transmogrify the opening version to the closing
version, because the backref search is hardwired independently of
ordinary variable matching. And that's because Perl doesn't instantiate
C<$1> soon enough. And that's because Perl relies on variable
interpolation to get subexpressions into regexes. Which leads us to...

=head2 Too hard to match a literal string

Since regexes undergo an interpolation pass before they're compiled,
anything you interpolate is forced to be treated as a regular
expression. Often that's not what you want, so we have the klunky
C<\Q$string\E> mechanism to hide regex metacharacters. And that's
because...

=head2 Two-level interpretation is problematic

The problem with C<\Q$string\E> arises because of the fundamental
mistake of using interpolation to build regexes instead of letting the
regex control how it treats the variables it references. Regexes aren't
strings, they're programs. Or, rather, they're strings only in the
sense that any piece of program is a string. Just as you have to work
to eval a string as a program, you should have to work to eval a string
as a regular expression. Most people tend to expect a variable in a
regular expression to match its contents literally. Perl violates that
expectation. And because it violates that expectation, we can't make
C<$1> synonymous with C<\1>. And interpolated parentheses throw off the
capture count, so you can't easily use interpolation to call subrules,
so we invented C<(??{$var})> to get around that. But then you can't
actually get at the parentheses captured by the subrule. The
ramifications go on and on.

=head2 Too little abstraction

Historically, regular expressions were considered a very low-level
language, a kind of glorified assembly language for the regex engine.
When you're only dealing with ASCII, there is little need for
abstraction, since the shortest way to say C<[a-z]> is just that. With
the advent of the eighth bit, we started getting into a little bit of
trouble, and POSIX started thinking about names like C<[:alpha:]> to
deal with locale difficulties. But as with the problem of conciseness,
the culture was still biased away from naming abstractly anything that
could be expressed concretely.

However, it's almost impossible to write a parser without naming
things, because you have to be able to name the separate grammar rules
so that the various rules can refer to each other.

It's difficult to deal with any subset of Unicode without naming it.
These days, if you see C<[a-z]> in a program, it's probably an outright
bug. It's much better to use a named character property so that your
program will work right in areas that don't just use ASCII.

Even where we do allow names, it tends to be awkward because of the
cultural bias against it. To call a subrule by name in Perl 5 you have
to say this:

    (??{$rule})

That has 4 or 5 more characters than it ought to. Dearth of abstraction
produces bad Huffman coding.

=head2 Little support for named captures

Make that "no support" in Perl, unless you include assignment to a
list. This is just a part of the bias against naming things. Instead we
are forced to number our capturing parens and count. That works okay
for the top-level regular expression, when we can do list assignment or
assign C<$1> to C<$foo>. But it breaks down as soon as you start trying
to use nested regexes. It also breaks down when the capturing
parentheses match more than once. Perl handles this currently by
returning only the last match. This is slightly better than useless,
but not by much.

=head2 Difficult to use nested patterns

For many of the reasons we've mentioned, it's difficult to make regexes
refer to each other, and even if you do, it's almost impossible to get
the nested information back out of them. And there are entire classes
of parsing problems that are not solvable without recursive
definitions.

=head2 Little support for grammars

Even if it were easier for regexes to refer to other regexes, we'd
still have the problem that those other regexes aren't organized in any
meaningful way. They might be off in variables that come and go at the
whim of the surrounding context.

When we have an organized system of parsing rules, we call it a
grammar. One advantage of having a grammar is that you can optimize
based on the assumption that the rules maintain their relationship to
each other. For instance, if you think of grammar rules as a funny kind
of subroutine, you can write an optimizer to inline some of the
subrules--but only if you know the subrule is fixed in the grammar.

Without support for grammar classes, there's no decent way to think of
deriving one grammar from another. And if you can't derive one grammar
from another, you can't easily evolve your language to handle new kinds
of problems.

=head2 Inability to define variants

If we want to have variant grammars for Perl dialects, then what about
regex dialects? Can regexes be extended either at compile time or at
run time? Perl 5 has some rudimentary overloading magic for rewriting
regex strings, but that's got the same problems as source filters for
Perl code; namely that you just get the raw regex source text and have
to parse it yourself. Once again the fundamental assumption is that a
regex is a funny kind of string, existing only at the behest of the
surrounding program.

Do we think of regexes as a real, living language?

=head2 Poor integration with rich languages

Let's face it, in the culture of computing, regex languages are mostly
considered second-class citizens, or worse. "Real" languages like C and
C++ will exploit regexes, but only through a strict policy of
apartheid. Regular expressions are our servants or slaves; we tell them
what to do, they go and do it, and then they come back to say whether
they succeeded or not.

At the other extreme, we have languages like Prolog or Snobol where the
pattern matching is built into the very control structure of the
language. These languages don't succeed in the long run because
thinking about that kind of control structure is rather difficult in
actual fact, and one gets tired of doing it constantly. The path to
freedom is not to make everyone a slave.

However, I would like to think that there is some happy medium between
those two extremes. Coming from a C background, Perl has historically
treated regexes as servants. True, Perl has treated them as trusted
servants, letting them move about in Perl society better than any other
C-like language to date. Nevertheless, if we emancipate regexes to
serve as co-equal control structures, and if we can rid ourselves of
the regexist attitudes that many of us secretly harbor, we'll have a
much more productive society than we currently do. We need to empower
regexes with a sense of control (structure). It needs to be just as
easy for a regex to call Perl code as it is for Perl code to call a
regex.

=head2 Missing backtracking controls

Perl 5 started to give regexes more control of their own destiny with
the "grab" construct, C<< (?>...) >>, which tells the regex engine that
when it fails to match the rest of the pattern, it should not backtrack
into the innards of the grab, but skip back to before it. That's a
useful notion, but there are problems. First, the notation sucks, but
you knew that already. Second, it doesn't go far enough. There's no way
to backtrack out of just the current grouping. There's no way to
backtrack out of just the current rule. Both of these are crucial for
giving first-class status to the control flow of regexes.

=head2 Difficult to define assertions

Notionally, a regex is an organization of assertions that either
succeed or fail. Some assertions are easily expressed in traditional
regex language, while others are more easily expressed in a procedural
language like Perl.

The natural (but wrong) solution is to try to reinvent Perl expressions
within regex language. So, for instance, I'm rejecting those RFCs that
propose special assertion syntax for numerics or booleans. The better
solution is to make it easier to embed Perl assertions within regexes.

=head1 Brave New World

I've just made a ton of negative assertions about the current state of
regex culture. Now I'd like you to perform a cool mental trick and turn
all those negatives assertions into positive assertions about what I'm
going to say, because I'm not intending to give the rationale again,
but just present the design as it stands. Damian will discuss an
extended example in his Exegesis 5, which will show the big picture of
how these various features work together to produce a much more
readable whole.

So anyway, here's what's new.

=head2 Metacharacter Reform

Some things stay the same: C<(...)> captures text just as it did
before, and the quantifiers C<*>, C<+>, and C<?> are also unchanged.
The vertical bar C<|> still separates alternatives. The backslash C<\>
still protects the following character from its ordinary
interpretation. The C<?> suffix character still does minimal matching.
(Note that these are by far the most commonly used metacharacters, so
many ordinary regexes will look nearly identical in Perl 5 and Perl 6.)

Since C</x> extended syntax is now the default, C<#> is now always a
metacharacter indicating a comment, and whitespace is now always
"meta". Whitespace is now the standard way to separate regex tokens
that would otherwise be confused as a single token.

Even in character classes, whitespace is not taken literally any more.
Backwhack the space if you mean it literally, or use C<< <sp> >>,
or C<\040>, or C<\x20>, or C<\c[SPACE]>. But speaking of character
classes...

Perhaps the most radical change is that I've taken C<[...]> away from
character classes and made it the non-capturing grouping operator,
because grouping is more fundamental than character classes, and
explicit character classes are becoming less common than named
character classes. (You can still do character classes, just not with
bare square brackets.)

I've also stolen C<{...}> from generalized quantifiers and made them
into closure delimiters. (Use C<< <n,m> >> for the generalized
quantifier now.)

[Update: The generalized quantifier is now C<**{n..m}>.]

I've stolen three new metacharacters. The new extensible metasyntax for
assertions uses angle brackets, C<< <...> >>. And the colon C<:> is
now used for declaration and backtracking control. (Recall Larry's 2nd
Law of Language Redesign: Larry gets the colon.) The colon always
introduces a token that controls the meaning of what is around it. The
nature of the token depends on what follows the colon. Both the colon
syntax and angle syntax are extensible. (Backslash syntax is also
extensible.)

This may sound like we're complexifying things, but we're really
simplifying. We now have the following regex invariants:

    (...)       # always delimits a capturing group
    [...]       # always delimits a non-capturing group
    {...}       # always delimits a closure
    <...>       # always delimits an assertion
    :...        # always introduces a metasyntactic token

(Note that we're using "assertion" here in the broad sense of anything
that either matches or fails, whether or not it has a width.)

[Update: We now also have C<«...»> for non-capturing assertions.]

The nature of the angle assertion is controlled by the first character
inside it. If the first character is alphabetic, it's a grammatical
assertion, and the entire first word controls the meaning. The word is
first looked up in the current grammar, if any. If not found there, it
is checked to see if it is one of the built-in grammar rules such as
those defined by the Unicode property classes. If the first character
is not alphabetic, there will be special rules in the current grammar
or in the Perl grammar for looking up the parse rule. For instance, by
default, any assertion that begins with C<!> is simply negated.
Assertions that start with a digit are assumed to be a range assertion
(C<< <n,m> >>) regarding the previous atom. (Taking the last two
together, you can say C<< <!n,m> >> to exclude a range.) Assertions
that start with C<$>, C<@>, C<%>, or C<&> are assumed to interpolate an
indirect regex rule stored in a variable or returned by a subroutine.
An assertion that starts with a parenthesis is a closure being used as
an assertion. An assertion that starts with a square bracket or another
angle bracket is a character class. An assertion that starts with a
quote asserts the match of a literal string. And so on.

[Update: If the inside character is an angle bracket, it's the ASCII
workaround C<<< <<...>> >>> for C<«...»>.]

Some metacharacters are still used but have a slightly different
meaning, in part to get rid of the C</s> and C</m> modifiers, and in
part because most strings in Perl 6 will come from the filehandle
pre-chomped. So anchors C<^> and C<$> now always mean the I<real>
beginning and ending of the string. Use C<^^> and C<$$> to match the
beginnings and endings of lines I<within> a string. (They're doubled
because they're "fancier", because they can match in multiple places,
and because they'll be rarer, so Huffman says they should be longer.)
The C<^^> and C<$$> also match where C<^> and C<$> would.

[Update: If the string ends with a newline, the null string following it
is not considered a line.  So neither C<^^> nor C<$$> match after a final
newline.  In other words, C<$$> matches at C<$> only if there is "extra"
text there not terminated by a newline.]

The dot C<.> now always matches any character including newline. (Use
C<\N> to match a non-newline. Or better, use an autochomping
filehandle, if you're processing line-by-line.)

In a sense, the sigils C<$>, C<@>, C<%>, and C<&> are different
metacharacters because they don't interpolate, but are now subject to
the interpretation of the regex engine. This allows us to change the
default behavior of ordinary "interpolation" to a literal match, and
also lets us put in lvalue-ish constructs like:

    / $name := (\S+) /
    / @kids := [(\S+) \s+]* /
    / %pets := [(\S+) \: (\S+) \s+]* /

(Notice also the delicate interplay of quantified non-capturing
brackets with capturing parens, particularly for gathering multiple
values or even multiple key/value pairs.)

Here are some of the metacharacter differences in table form:

    Old                   New
    ---                   ---
    /pat pat #text        /pat pat #text
        pat/x                 pat/              # Look Ma, no /x!
    /patpat(?#text)/       /pat pat <('text')>/ # can always use whitespace

    /pat pat/             / pat\ pat /          # match whitespace literally
                          / pat \s* pat /       # or generically
                          / pat \h* pat /       # or horizontally
                          / pat <' '> pat /     # or as a literal string
                          / pat <sp> pat /      # or by explicit rule
                          /:w pat pat/          # or by implicit rule

    /^pat$/               /^pat\n?$/            # ^ and $ mean string
    /^pat$/m              /^^pat$$/             # no more /m
    /\A...(^pat$)*...\z/m /^...(^^pat$$)*...$/  # no more \A or \z
    /.*\n/                /\N*\n/               # \N is negated \n
                          /.*?\n/               # this still works
    /.*/s                 /.*/                  # . always matches "any"

    \Q$string\E           $string               # interpret literally

    (?{ code })           { code }              # call code, ignore return
                          { code or fail }      # use code as an assertion

    (??{$rule})           <$var>                # call $var as regex
                          <name>                # call rule from current grammar

[Update: Follows ordinary scoping rules, so lexical rules take precedence
over grammar rules.]

                          <Other::rule>         # call rule from some Other grammar
                          <*rule>               # bypass local rule to call built-in
                          <@array>              # call array of alternate rules
                          <%hash>               # parse keyword as key to rule
                          <@array[1]>           # call a rule from an array
                          <%hash{"x"}>          # call a rule from a hash
                          <&sub(1,2,3)>         # call a rule returned by a sub
                          <{ code }>            # call return value as anonymous rule
                          <( code )>            # call code as boolean assertion

                          <name(expr)>          # call rule, passing Perl args
                          { .name(expr) }       # same thing.

                          <$var(expr)>          # call rule indirectly by name
                          { .$var(expr) }       # same thing.

                          <name pat>            # call rule, passing regex arg
                          { .name(/pat/) }      # same thing.

                          # maybe...
                          <name: text>          # call rule, passing string
                          { .name(q<text>) }    # same thing.

    [\040\t\p{Zs}]        \h                    # horizontal whitespace
    [\r\n\ck\p{Zl}\p{Zp}] \v                    # vertical whitespace
    [a-z]                 <[a-z]>               # equivalently non-international
                          <alpha>               # more international
    [[:alpha:][:digit:]]  <<alpha><digit>>      # POSIX classes are built-in rules

[Update: That must now be written C<<< <+<alpha>+<digit>> >>>, or it
will be mistaken for C<< «alpha><digit» >>, which doesn't work too well.]

[Update: Actually, that's now written C<< <+alpha+digit> >>, avoiding
the mistaken impression entirely.]

    {n,m}                 <n,m>                 # assert repeat count

[Update: Is now C< **{n..m} >.]

    {$n,$m}               <$n,$m>               # indirect repeat counts

[Update: Is now C< **{$n..$m} >.]

    (?>.*)                [.*]:                 # don't backtrack through [.*]
                          .*:                   # brackets not necessary on atom
                          (.*):                 # same, but capture
                          <xyz>:                # don't backtrack into subrule

                          :                     # skip previous atom when backtracking
                          ::                    # fail all |'s when backtracking
                          :::                   # fail current rule when backtracking

                          :=                    # bind a name to following atom
    my ($x) = /(.*)/      my $x; / $x:=(.*) /   # may now bind it inside regex

    (?i)                  :i                    # ignore case in the following
                          :ignorecase           # same thing, self-documenting form
    (?i:...)              [:i ...]              # can limit scope without capture
                          (:i ...)              # can limit scope with capture

Declarations like C<:i> are lexically scoped and do not pass to any
subrules. Each rule maintains its own sensitivity. There is no built-in
operator to turn case ignorance back off--just call a different rule
and it's automatically case sensitive again. (If you want a
parameterized subrule, that can be arranged. It's just a method, after
all. Proof of this assertion is left to future generations of hackers.)

=head2 Backslash Reform

There are some changes to backslash sequences. Character properties
C<\p> and C<\P> are no longer needed--predefined character classes are
just considered intrinsic grammar rules. (You can negate any
C<< <...> >> assertion by using C<< <!...> >> instead.) As
mentioned in a previous Apocalypse, the C<\L>, C<\U>, and C<\Q>
sequences no longer use C<\E> to terminate--they now require bracketing
characters of some sort. And C<\Q> will rarely be needed due to regex
policy changes. In fact, they may all go away since it's easy to say
things like:

    $(lc $foo)

[Update: And in fact, they did all go away.]

[Update: C<$()> is gone, so you have to say C<< <{lc $foo}> >> or some such.]

For any bracketing construct, square brackets are preferred, but others
are allowed:

    \x[...]     # preferred, indicates simple bracketing
    \x(...)     # okay, but doesn't capture.
    \x{...}     # okay, but isn't a closure.
    \x<...>     # okay, but isn't an assertion

The C<\c> sequence is now a bracketing construct, having been extended
from representing control characters to any named character.

Backreferences such as C<\1> are gone in favor of the corresponding
variable C<$1>. C<\A>, C<\Z>, and C<\z> are gone with the disappearance
of C</s> and C</m>. The position assertion C<\G> is gone in favor of a
C<:c> modifier that forces continuation from where the last match left
off. That's because C<\G> was almost never used except at the front of
a regex. In the unlikely event that you want to assert that you're at
the old final position elsewhere in your regex, you can always test the
current position (via the C<.pos> method) with an assertion:

    $oldpos = pos $string;
    $string =~ m/... <( .pos == $oldpos )> .../;

You may be thinking of C<.pos> as the final position of the previous
match, but that's not what it is. It's the I<current> position of the
current match. It's just that, between matches, the current position of
the current match happens to be the same as the final position of the
current match, which happens to be the last match, which happens to be
done. But as soon as you start another match, the last match is no
longer the current match.

Note that the C<:c> continuation is needed only on constructs that
ordinarily force the search to start from the beginning. Subrules
automatically continue at the current location, since their initial
position is controlled by some other rule.

[Update: There's also a C<:p> that is like C<:c> but anchors the
next chunk to the starting position, so it's good for parsers.]

There are two new backslash sequences, C<\h> and C<\v>, which match
horizontal and vertical whitespace respectively, including Unicode
spacing characters and control codes. Note that C<\r> is considered
vertical even though it theoretically moves the carriage sideways.
Finally, C<\n> matches a logical newline, which is not necessarily a
linefeed character on all architectures. After all, that's why it's an
"n", not an "l". Your program should not break just because you
happened to run it on a file from a partition mounted from a Windows
machine. (Within an interpolated string, C<\n> still produces whatever
is the normal newline for the current architecture.)

    Old                 New
    ---                 ---
    \x0a                \x0a                    # same
    \x{263a}            \x263a                  # brackets required only if ambiguous
    \x{263a}abc         \x[263a]abc             # brackets required only if ambiguous
    \0123               \0123                   # same (no ambiguity with $123 now)
    \0123               \0[123]                 # can use brackets here too

    \p{prop}            <prop>                  # properties are just grammar rules
    \P{prop}            <!prop>

[Update: That's C<< <-prop> >> now, since C<< <!prop> >> would be a zero-width
assertion.]

    [\040\t\p{Zs}]      \h                      # horizontal whitespace
    space               \h                      # not exact, but often more correct
    [\r\n\ck\p{Zl}\p{Zp}] \v                    # vertical whitespace

    \Qstring\E          \q[string]
                        <'string with spaces'>  # match literal string
                        <' '>                   # match literal space

    \E                  gone                    # use \Q[...] instead

[Update: C<\Q[...]> is also gone.  Use C<< <{quotemeta ...}> >>.]

    \A                  ^                       # ^ now invariant
    \a                  \c[BEL]                 # alarm (bell)

    \Z                  \n?$                    # clearer
    \z                  $                       # $ now invariant
    \G                  <( .pos == $oldpos )>   # match at particular position
                                                # typically just use m:c/pat/

[Update: Or C<m:p/pat/>.]

    \N{CENT SIGN}       \c[CENT SIGN]           # named character
    \c[                 \e                      # escape
    \cX                 \c[^X]                  # control char
    \n                  \c[LF]                  # specifically a linefeed
    \x0a\x0d            \x[0a;0d]               # CRLF
    \x0a\x0d            \c[CR;LF]               # CRLF (conjectural)
    \C                  [:u0 .]                 # forced byte in utf8 (dangerous)

[Update: The Unicode levels are now specified by C<:bytes>, C<:codes>,
C<:graphs>, and C<:langs>, not by numeric level.]

    [^\N[CENT SIGN]]    \C[CENT SIGN]           # match any char but CENT SIGN

    \Q$var\E            $var                    # always assumed literal,
    \1                  $1                      # so $1 is literal backref
    /$1/                my $old1 = $1; /$old1/  # must use temporary here

    \r?\n               \n                      # \n asserts logical newline

    [^\n]               \N                      # not a logical newline
                        \C[LF]                  # not a linefeed

    [^\t]               \T                      # not a tab (these are conjectural)
    [^\r]               \R                      # not a return
    [^\f]               \F                      # not a form feed
    [^\e]               \E                      # not an escape
    [^\x1B]             \X1B                    # not specified hex char
    [^\x{263a}]         \X[263a]                # not a Unicode SMILEY

    \X                  <.>                     # a grapheme (combining char seq)
                        [:u2 .]                 # At level 2+, dot means grapheme

[Update: That's C<:graphs> now.]

Under level 2 Unicode support, a character is assumed to mean a
grapheme, that is, a sequence consisting of a base character followed
by 0 or more combining characters. That not only affects the meaning of
the C<.> character, but also any negated character, since a negated
character is really a negative lookahead assertion followed by the
traversal of a single character. For instance, C<\N> really means:

    [<!before \n> . ]

So it doesn't really matter how many characters C<\n> actually matches.
C<\N> always matches a single character--whatever that is...

=head2 Modifier Reform

You can't use colon for a regex delimiter any more. That's because
regex modifiers may now be placed in front of a regex construct:

    s:w:i:e /foo/bar/           # :words :ignorecase :each

That can also be written:

    s/:w:i:e foo/bar/           # :words :ignorecase :each

Single character modifiers may be bundled like this:

    s:wie /foo/bar/             # :words :ignorecase :each

[Update: Bundling is no longer allowed. Whitespace is allowed.]

...but only if the sequence as a whole is not already defined as a long
modifier, since ambiguity will be resolved in favor of the long
modifier. Long modifiers may I<not> be bundled with any other modifier.
So this is legal:

    s:once:wie /foo/bar/

[Update: Not any more.]

but not these (unless you've defined them):

    s:wieonce /foo/bar/
    s:oncewie /foo/bar/

Not only is colon disallowed as a regex delimiter, but you may no
longer use parentheses as the delimiters either. This will allow us to
parameterize modifiers:

    s:myoption($x) /foo/bar/

This rule also allows us to differentiate C<s///> from an C<s()>
function, C<tr///> from C<tr()>, etc. If you want matching brackets for
the delimiters I'd suggest that you use square brackets, since they now
mean grouping without capturing.

[Update: Parens may be used for delimiters as long as they are not
adjacent to the final option.  They may be separated with either a
colon or whitespace.]

Several modifiers, C</x>, C</s>, and C</m>, are no longer needed and
have been retired. It's unclear whether C</o> is necessary any more. We
will assume it's gone unless it's shown that caching can't handle the
problem. Note that the regex now has more control over when to cache
subrules because it is no longer subject to the vagaries of standard
interpolation.

The old C</c> modifier is gone because regexes never reset the position
on failure any more. To do that, set C<$string.pos = 0> explicitly. But
note also that assigning to a string automatically resets its position
to C<0>, so any string in your typical loop is going to start with its
current search position already set C<0>. Modifying a string in place
causes the position to move to the end of the replacement section by
default, if the position was within the span replaced. (This is
consistent with C<s///> semantics.)

The C</e> modifier is also gone, since it did reverse parsing magic,
and C<:e> will be short for C<:each>--see below. It's still easy to
substitute the value of an expression though:

    s/pat/$( code )/;

or even

    s(/pat/, { code });

[Update: C<:each> is gone, and we're back to C<:global> and C<:g> to
stay with common culture.  Since C<$()> is also gone, just interpolate
code using a closure: S<C<s/pat/{ code }/>>.]

There's a new modifier, C<:once>, that causes a match to succeed only
once (like the old C<?...?> construct). To reset it, use the C<.reset>
method on the regex object. (If you haven't named the regex object, too
bad...)

Another new modifier is C<:w>, which causes an implicit match of
whitespace wherever there's literal whitespace in a pattern. In other
words, it replaces every sequence of actual whitespace in the pattern
with a C<\s+> (between two identifiers) or a C<\s*> (between anything
else). So

    m:w/ foo bar \: ( baz )*/

really means (expressed in Perl 5 form):

    m:p5/\s*foo\s+bar\s*:(\s*baz\s*)*/

[Update: The optional whitespace is actually matched by the C<< <?ws> >>
rule, which you can redefine to change the semantics of C<:words>.]

You can still control the handling of whitespace under C<:w>, since we
extend the rule to say that any explicit whitespace-matching token
can't match whitespace implicitly on either side. So:

    m:w/ foo\ bar \h* \: (baz)*/

really means (expressed in Perl 5 form):

    m:p5/\s*foo bar[\040\t\p{Zs}]*:\s*(baz)*/

The first space in

    /[:w foo bar]/

matches C<\s*> before "C<foo>". That's usually what you want, but if
it's not what you want, you have a little problem. Unfortunately you
can't just say:

    /[:wfoo bar]/

That won't work because it'll look for the C<:wfoo> modifier. However,
there are several ways to get the effect you want:

    /[:w()foo bar]/ 
    /[:w[]foo bar]/ 
    /[:w\bfoo bar]/ 
    /[:w::foo bar]/

That last one is just our friend the C<::> operator in disguise. If you
backtrack into it, you're leaving the brackets anyway, so it's
essentially a no-op.

The new C<:c>/C<:cont> modifier forces the regex to continue at the
current "pos" of the string. It may only be used outside the regex.
(Well, it could be used inside but it'd be redundant.) The modifier
also forces the regex to match only the next available thing. That's
not quite the same as the C<^> anchor, though, because it not only
disables the implicit scanning done by C<m//> and C<s///>, but it also
works on more than the first iteration. It forces all matches to be
contiguous, in other words. So C<:c> is short for both "continue" and
"contiguous". If you say

    $_ = "foofoofoo foofoofoo";
    s:each:cont/foo/FOO/;

you get:

    FOOFOOFOO foofoofoo

This may seem odd, but it's precisely the semantics of any embedded
regex:

    $_ = "foofoofoo foofoofoo";
    $rx = rx/foo/;
    m/<$rx>*/;          # matches "foofoofoo"

[Update: Now we use C<:p>/C<:pos> to get contiguous semantics.
The C<:c>/C<:continue> modifier starts scanning again each time, so
when used with C<:global> it really only influences where the first
match is found.]

A modifier that starts with a number causes the pattern to match that
many times. It may only be used outside the regex. It may not be
bundled, because ordinals are distinguished from cardinals. That is,
how it treats those multiple matches depends on the next character. If
you say

    s:3x /foo/bar/

then it changes the first 3 instances. But if you say

    s:3rd /foo/bar/

it changes only the 3rd instance. You can say

    s:1st /foo/bar/

but that's just the default, and should not be construed as equivalent
to C<:once>, which matches only once, ever. (Unless you C<.reset> it,
of course.)

You can combine cardinals and ordinals:

    s:3x:3rd /foo/bar/

That changes the 3rd, 6th, and 9th occurrences. To change every other
quote character, say

    s:each:2nd /"/\&rquot;/;

C<:each> is synonymous with C<:3x> (for large values of C<3>). Note
that C<:each> does not, in fact, generate every possible match, because
it disallows overlaps. To get every possible match, use the C<:any>
modifier. Saying:

    $_ = "abracadabra";
    @all = m:any /a[^a]+a/;

produces:

    abra aca ada abra

It can even match multiple times at the same spot as long as the rest
of the regex progresses somehow. Saying:

    @all = m:any /a.*?a/;

produces:

    abra abraca abracada abracadabra aca acada acadabra ada adabra abra

If you say

    $sentence.m:any /^ <english> $/

you'll get every possible parsing of the sentence according to the
rules of C<english> (not to be confused with the rules of English,
which are already confusing enough, except when they aren't).

[Update: C<:any> has been replaced with C<:overlap> and C<:exhaustive>
to differentiate the cases of wanting one solution at each position from
wanting all solutions at every position.]

To indicate varying levels of Unicode support we have these modifiers,
which may be used either inside or outside a regex:

    :u0         # use bytes       (. is byte)
    :u1         # level 1 support (. is codepoint)
    :u2         # level 1 support (. is grapheme)
    :u3         # level 1 support (. is language dependent)

These modifiers say nothing about the state of the data, but in general
internal Perl data will already be in Normalization Form C, so even
under C<:u1>, the precomposed characters will usually do the right
thing. Note that these modifiers are for overriding the default support
level, which was probably set by pragma at the top of the file.

[Update: Replaced with C<:bytes>, C<:codes>, C<:graphs>, and C<:langs>.]

Finally, there's the C<:p5> modifier, which causes the rest of the
regex (or group) to be parsed as a Perl 5 regular expression, including
any interpolated strings. (But it still doesn't enable Perl 5's
trailing modifiers.)

[Update: Now spelled out C<:perl5>.]

    Old                 New
    ---                 ---
    ?pat?               m:once/pat/             # match once only
    /pat/i              m:i/pat/                # ignorecase
                        /:i pat/                # ignorecase
    /pat/x              /pat/                   # always extended
    /pat\s*pat/         /:w pat pat/            # match word sequence
    /(?i)$p5pat/        m:p5/(?i)$p5pat/        # use Perl 5 syntax
    $n = () = /.../g    $n = +/.../;            # count occurrences
    for $i (1..3){s///} s:3///;                 # do 3 times
    /^pat$/m            /^^pat$$/               # no more /m
    /./s                /./                     # no more /s
    /./                 /\N/                    # . now works like /s

=head2 Keyword and Context Reform

Deferred regex rules are now defined with C<rx//> rather than C<qr//>,
because a regular expression is no longer a kind of quoted string.

Actually, just as you can define closures without an explicit C<sub>,
any C<//> or C<rx//> declares a deferred regex if it's not in a context
that executes it immediately. A regex is executed automatically if it's
in a boolean, numeric, or string context. But assignment to an untyped
variable is not such a context, nor is passing the regex as an untyped
parameter to a function. (Of course, an explicitly declared RULE
parameter doesn't provide an evaluating context either.)

So these are equivalent:

    my $foo = /.../;            # create regex object
    my $foo = rx[...];          # create regex object
    my $foo = rule {...};       # create regex object

Likewise, these are equivalent:

    @x = split /.../;
    @x = split rx[...];
    @x = split rule {...};

[Update: Note that C<split> supplies an implicit C<:c> here.]

The "rule" syntax is just a way of declaring a deferred regex as if it
were a subroutine or method. More on that later.

To force an immediate evaluation of a deferred regex where it wouldn't
ordinarily be, you can use the appropriate unary operator:

    my $foo = ?/.../;   # boolean context, return whether matched, 
    my $foo = +/.../;   # numeric context, return count of matches
    my $foo = _/.../;   # string context, return captured/matched string

[Update: Unary C<_> is now unary C<~>.]

The standard match and substitution forms also force immediate
evaluation regardless of context:

    $result = m/.../;           # do match on topic string
    $result = s/.../.../;       # do substitution on topic string

These forms also force the regex to start matching at the beginning of
the string in question and scan forward through the string for the
match, as if there were an implicit C<.*?> in front of every iteration.
(Both of these behaviors are suppressed if you use the C<:c>/C<:cont>
modifier). In contrast, the meaning of the deferred forms is dependent
on context. In particular, a deferred regex naturally assumes C<:c>
when used as a subrule. That is, it continues where the last match left
off, and the next thing has to match right there at the head of the
string.

[Update: A regex actually assumes C<:p> semantics when used as a subrule.
And the long form of C<:c> is C<:continue>.]

In any other context, including list context, a deferred regex is
I<not> immediately evaluated, but produces a reference to the regex
object:

    my $rx = /.../;     # not evaluated
    my @foo = $rx;      # ERROR: type mismatch.
    my @foo = ($rx);    # One element, a regex object.
    my @foo = (/.../);  # Same thing.
    my @foo := $rx;     # Set autogrow rule for @foo.

To evaluate repeatedly in list context, treat the regex object as you
would any other iterator:

    my @foo = <$rx>;

You can also use the more explicit form:

    my @foo = m/<$rx>/;

Those aren't identical, since the former assumes C<:c> and starts up at
the current position of the unmentioned topic, while the latter
explicitly resets the position to the beginning before scanning. Also,
since the deferred regex assumes a C<:c> modifier, C<< <$rx> >>
won't scan through the string like C<m//>. It can return multiple
values to the list, but they have to be contiguous. You can get the
scanning effect of C<m//> by prepending the pattern with C<.*?>.

[Update: Where the preceding paragraphs says C<:c> read C<:p>.]

But it's vitally important to understand this fundamental change, that
C<//> is no longer a short form of C<m//>, but rather a short form of
C<rx//>. If you want to add modifiers to a C<//>, you have to turn it
into an C<rx//>, not an C<m//>. It's now I<wrong> to call C<split> like
this:

    split m/.../

(That is, it's wrong unless you actually want the return value of the
pattern match to be used as the literal split delimiter.)

The old C<?...?> syntax is gone. Indeed, it has to go for us to get the
unary C<?> operator.

    Old                 New
    ---                 ---
    ?pat?               m:once/pat/
    qr//                rx//
                        rule { }

=head2 Null String Reform

The null pattern is now illegal. To match whatever you used to match
with a null pattern, use one of these:

    Old                 New
    ---                 ---
    //                  /<prior>/       # match what prior match did
    //                  /<null>/        # match the null string between chars
    (a|b|)              (a|b|<null>)    # match a null alternative

Note that, as an assertion, C<< <null> >> always succeeds. You never
want to say:

    / <null> | single | double | triple | home run /

because you'll never get to first base.

=head2 Extension Syntax Reform

There are no longer any C<(?...)> sequences, because parens now always
capture. Some of the replacement sequences take their intrinsic
scoping from C<< <...> >>, while others are associated with other
bracketing characters, or with any arbitrary atom that could be a
bracketed construct. Looking at the metasyntax problem from the
perspective of a Perl5-to-Perl6 translator, here's what the various
Perl 5 extension constructs translate to:

    Old                 New
    ---                 ---
    (??{$rule})         <$rule>         # call regex in variable
    (?{ code })         { code }        # call Perl code, ignore result
    (?#...)             <('...')>       # in-line comment, rarely needed
    (?:...)             [...]           # non-capturing brackets
    (?=...)             <before ...>    # positive lookahead
    (?!...)             <!before ...>   # negative lookahead
    (?<=...)            <after ...>     # positive lookbehind
    (?<!...)            <!after ...>    # negative lookbehind
    (?>...)             [...]:          # grab (any atom)

    (?(cond)yes|no)     [ cond :: yes | no ]
    (?(1)yes|no)        [ <(defined $1)> :: yes | no ]

The C<< <$rule> >> construct does a "delayed" call of another
regular expression stored in the C<$rule> variable. If it is a regex
object, it's just called as if it were a subroutine, so there's no
performance problem. If it's a string, it is compiled as a regex and
executed. The compiled form is cached as a property of the string, so
it doesn't have to be recompiled unless the string changes. (This
implies that we can have properties that invalidate themselves when
their base object is modified.) In either case, the evaluated regex is
treated as a subrule, and any captures it does are invisible to the
outer regex unless the outer regex takes steps to retrieve them. In any
event, subrule parens never change the paren count of the outer rule.

[Update: Use C<«$rule»> to turn any capturing rule into a non-capturing
rule.  Or say C<(«$rule»)> if you only want to capture the string.]

The C<{code}> form doesn't return anything meaningful--it is used for
its side effects. Any such closure may behave as an assertion. It
merely has to throw an exception in order to fail. To throw such an
exception (on purpose) one may use C<fail>:

    $_ = "666";
    / (\d+) { $1 < 582 or fail }/

As with any assertion, the failing closure starts backtracking at the
location of the closure. In this case, it backtracks into the C<\d+>
and ends up matching "C<66>" rather than "C<666>". If you didn't want
that, use C<\d+:> instead.

It's more succinct, however, to use the code assertion syntax. Just put
angles around a parenthesized Perl expression:

    / (\d+) <( $1 < 582 )> /

I find the parens to be vaguely reminiscent of the parentheses you have
to put around conditionals in C (but not Perl (anymore)). Also, the
parentheses are meant to remind you that you only want to put an
expression there, not a full statement.

Don't use a bare closure to try to interpolate a calculated regex,
since the result will be ignored. Instead, use the C<< <{expr}> >>
form to do that. As with C<< <&rule()> >>, the result will be
interpreted as a subrule, not as if it were interpolated.

Since a string is usually true, you can just assert it to get the
effect of an inline comment: C<< <("this is a comment")> >>. But I've
never used one except to show it as an example. Line ending comments
are usually much clearer. (Just bear in mind you can't put the final
regex delimiter on the same line, because it won't be seen in the
comment.) You could also use the C<{'...'}> construct for comments,
but then you risk warnings about "useless use of a string in void
context".

The C<[...]> is the new non-capturing bracket notation. It seems to
work very well for this purpose--I tried the other brackets and they
tend to "disappear" faster than square brackets. So we reserve C<(...)>
and C<< <...> >> for constructs where the visual distance between
opening and closing is typically shorter than for square brackets or
curlies. The square brackets also work nicely when lined up vertically
with vertical bars. Here's a declaration of a named rule from the class
Perl6Grammar. It parses Perl 6 statements. (Think of it as a funny
looking method declaration.)

    rule state  { <label>
                    [ <control>          {.control}
                    | <sideff> <eostate> {.sideff}
                    | <@other_statements>
                    ]
                };

Huffman coding says that rarer forms should be longer, and that's the
case with the lookahead and lookbehind assertions, C<< <before ...> >>
and C<< <after ...> >>. (The negations are formed via the general 
C<< <!...> >> rule.) Note that these prepositions are interpreted as
assertions, not operations. For example, C<< <before X> >> is to be
read "Assert that we are before X" rather than "Look before where we
are for X".

The new C<:> operator replaces the C<< (?>...) >> construct. It
modifies whatever comes before it, much like C<*> does, so it's
naturally scoped if the preceding atom (or quantified atom) is a
bracketed construct. Parsers can use this every time they commit to the
parsing of a token or phrase to tell the regex engine that there's no
point in backtracking through the atom in question, so backtracking
will skip backwards over the atom and continue with some earlier branch
point. The following takes a long time to fail if it has to look at
every sequence of "C<a>" to see if there is a "C<b>" after it:

    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaac" =~ /^ a* b /

But we already know that the only possible match is the longest one. So
if you put in the colon, it fails in one pass because the C<*> grabs
everything and gives nothing back on backtracking.

    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaac" =~ /^ a*: b /

You can use colon on a longer sequence too. The following might match a
list of expressions separated by comma:

    / <expr> [ , <expr> ]*: /

It is an error to use C<:> on any atom that does no backtracking. This
will help to catch errors where you've forgotten to backslash a literal
colon in things like:

    /^From: (.*)/

Perl 6 has no need for a special conditional construct like Perl 5's
C<(?(cond)yes|no)>. That's because with a slight tweak, ordinary
alternation can do the same thing. That tweak is our next backtracking
modifier, the C<::> operator. If you backtrack across it, it fails all
the way out of the current list of alternatives. Consider an ordinary
list of alternatives:

    [ <A> <X> | <B> <Y> | <C> <Z> ]

The way the rules of backtracking work, if either C<< <A> >> or
C<< <X> >> fail, it backtracks to the next alternative. Likewise for
C<< <B> >> and C<< <Y> >>. In the case of C<< <C> >> or C<< <E> >>,
there is no next alternative, so it naturally fails out of the entire
construct. That's not how a conditional is supposed to work, because
in the conditional, only the condition determines which case is
executed. Once you've committed to a particular case, it has to stand
or fall as if the conditional hadn't been there. So all we need for
our purposes is to have is something that separates the assertions
that matter from those that don't. That's what C<::> does, and it
reads rather well as a "then", or as a "corresponds to". If you write

    [ <A> :: <X>
    | <B> :: <Y>
    | <C> :: <Z>
    ]

then the failure of C<< <A> >>, C<< <B> >>, or C<< <C> >>
proceeds to the next case (if any), while any failure in
C<< <X> >>, C<< <Y> >>, or C<< <Z> >> is guaranteed to
backtrack out of the front of the alternative list and revise a former
choice (just as the success of C<< <X> >>, C<< <Y> >>, or
C<< <Z> >> is guaranteed to "forward track" out of the end of the
alternative list and try to match more). It's a natural mapping to
existing regex semantics. Here's a more realistic example from the Perl
6 grammar. It parses statement modifiers. (The C<< <ws> >> rule
parses optional whitespace.)

    rule modifier { if     <ws> :: <expr> { .new_cond(0,$expr) }
                  | unless <ws> :: <expr> { .new_cond(1,$expr) }
                  | while  <ws> :: <expr> { .new_loop(0,$expr) }
                  | until  <ws> :: <expr> { .new_loop(1,$expr) }
                  | for    <ws> :: <expr> { .new_for($expr)   }
                  | <@other_modifiers>  # user defined
                  | <null>              # no modifier
                  }

In each case, once we recognize a keyword (and its following
whitespace), we need to look for an expression, and then call a closure
that builds the syntax tree. If either of those fails, the entire
modifier rule fails. We only get to the last two alternatives on
failure of assertions before the C<::>.

Note that the C<::> only says that we can't backtrack from the "then"
into the "if". It says nothing about backtracking into the alternative
list as a whole. The alternatives are still choice points, so the regex
engine is allowed to backtrack into the alternative list and try
another alternative. (To disable that, simply put a C<:> after the
closing bracket of the alternative list.)

There is nothing in Perl 5 corresponding to the C<:::> operator, but it
works just like C<::>, only more so. If you backtrack across it, it
fails all the way out of the current rule definition (though not out of
any rule invoking this definition). That is, it fails all the way out
of the innermost lexically enclosing C</.../>, C<m/.../>, C<s/...//>,
C<rx/.../>, or C<rule {...}>, skipping out through any enclosing
nestings of C<< <...> >>, C<[...]>, or C<(...)>. (A pattern nested
within a closure is classified as its own rule, however, so it never
gets the chance to pass out of a C<{...}> closure.)

Since the alternatives in our last example are at the top level of the
regex, we could have used the C<:::> operator to get the same effect as
C<::>, because terminating the rule and terminating the alternation
amount to the same thing in that case. You can think of all of these as
variants on Prolog's "cut" operator.

If you backtrack over the C<::::> operator, it will delete your program
from the disk. C<;-)>

Actually, the real name of the real C<::::> operator is
C<< <commit> >>. It fails the entire match if you backtrack over
it, not just the current rule. That is, it fails all the way out of the
I<outermost> dynamically enclosing C</.../>, C<m/.../>, C<s/...//>,
C<rx/.../>, or C<rule {...}> that is executing on the current string.

There is one "cut" operator that is beyond C<< <commit> >>; it is
appropriately named C<< <cut> >>, for two reasons. First of all,
it's a real cut operator in that, if you backtrack over it, the current
match fails completely, just like C<< <commit> >>. But that's just
a side effect of the other reason, which is that C<< <cut> >> cuts
off the front of the string that you're currently matching on, turning
the current position into the new beginning of the string. When you're
matching on a potentially infinite string, it's important that you have
a way of discarding that part of the match that you've already
committed to. In Perl 5, the only way to do that was with a coordinated
system of C<s/^pat//> operations. With the C<< <cut> >> assertion,
however, you can just match normally, and cut at one spot in your
top-level rule when you reach an "accept" state.

In the realm of idle speculation, we could go as far as to define a
variant of C<< <cut> >> that would render C<s///> slightly
redundant:

    s/foo/bar/;
    m/foo <replace("bar")> /

Note that we don't need any special forms for controlling the scope of
a "fail" in a closure. Just prefix the closure with the appropriate
backtracking operator:

    / pattern ::: { code() or fail } /  # fails entire rule

=head2 Character Class Reform

As we mentioned earlier, character classes are becoming more like
standard grammar rules, because the definition of "character" is
getting fuzzier. This is part of the motivation for demoting enumerated
character classes and stealing the square brackets for another purpose.
Actually, for old times' sake you still use square brackets on
enumerated character classes, but you have to put an extra set of
angles around it. But this actually tends to save keystrokes when you
want to use any named character classes or Unicode properties,
particularly when you want to combine them:

    Old                 New
    ---                 ---
    [a-z]               <[a-z]>
    [[:alpha:]]         <alpha>
    [^[:alpha:]]        <-alpha>
    [[:alpha:][:digit:]] <<alpha><digit>>

The outer C<< <...> >> also naturally serves as a container for any
extra syntax we decide to come up with for character set manipulation:

    <[_]+<alpha>+<digit>-<Swedish>>

[Update: If the first thing in angle brackets is another angle bracket,
you have to put a C<+> between to avoid confusion with "Texas" quotes
emulating French quotes.]

=head2 State

[This section gets pretty abstruse. It's okay if your eyes glaze over.]

Every regex match maintains a state object, and any closure within the
regex is actually an anonymous method of that object, which means in
turn that the closure's topic is the current state object. Since a
unary dot introduces a method call on the current topic, it follows
that you can call any method in the state object that way:

    /(.*) { print .pos }/       # print current position

The state object may in fact be an instance of a grammar class. A
grammar object has additional methods that know how to build a parse
tree. Its rules also know how to refer to each other or to rules of
related grammars.

Note that C<$_> within the closure refers to this state object, not the
original search string. If you search on the state object, however, it
pretends that you wanted to continue the search on the original string.
If the internal search succeeds, the position of the external state is
updated as well, just as if the internal search had been a rule invoked
directly from the outer regex.

Because the state object is aware of how the tree is being built, when
backtracking occurs the object can destroy parts of the parse tree that
were conjectured in error. Because the grammar's action methods have
control of the regex state, they can access named fields in the regex
without having to explicitly pass them to the method call.

For instance, in our earlier example we passed C<$expr> explicitly to
build the parse tree, but the method can actually figure that out
itself. So we could have just written:

    rule modifier { if     <ws> :: <expr> { .new_cond(0) }
                  | unless <ws> :: <expr> { .new_cond(1) }
                  | while  <ws> :: <expr> { .new_loop(0) }
                  | until  <ws> :: <expr> { .new_loop(1) }
                  | for    <ws> :: <expr> { .new_for }
                  | <@other_modifiers>  # user defined
                  | <null>              # no modifier
                  },

See Variable Scoping below for where C<@other_modifiers> gets looked
up.

Within a closure, C<$_> represents the current state of the current
regex, and by extension, the current state of all the regexes
participating in the current match. (The type of the state object is
the current grammar class, which may be an anonymous type if the
current grammar has no name. If the regex is not a member of a grammar,
it's of type RULE.) Part of the state of the current regex is the
current node of the parse tree that is being built. When the current
regex succeeds, the state object becomes a result object, and is
returned to the calling regex. The calling regex can refer to the
returned object as a "hypothetical" variable, the name of which is
either implicitly generated from the name of the rule, or explicitly
bound using C<:=>. Through that variable you can get at anything
captured by the subrule. (That is what C<$expr> was doing earlier.)

[Update: Only rules of the form C<< <ident> >> are captured by default.
You can use C<:=> to force capture of anything else, or the C<:keepall>
adverb to capture everything else.]

When the entire match succeeds, the top-level node is returned as a
result object that has various values in various contexts, whether
boolean, numeric, or string context. The name of the result object is
C<$0>. The result object contains all the other information, such as
C<$1>, C<$2>, etc. Unlike C<$&> in Perl 5, C<$0> is lexically scoped to
the enclosing block. By extension, C<$1>, etc. are also lexically
scoped.

[Update: The C<$0> variable's functionality is now split up into a
C<$/> variable that represents the match state object, and a C<$0>
variable that represents the capture of the entire pattern.  In other
words, C<$0> is exactly like a C<$1> capture, only with assumed parens
around the entire pattern.]

As a kind of iterator, a regex stored in a variable doesn't expand in
list context unless you put angles around it or use it with C<m//>:

    $rx = /(xxx)/;
    print 1,2,<$rx($_)>;
    print 1,2,</(xxx)/>;
    my &rx := /(xxx)/;
    print 1,2,<rx($_)>;

C<$0>, C<$1>, etc. are not set in iterated cases like this. Each list
item is a result object, though, and you can still get at the internal
values that way.

=head2 Hypothetical Variables, er Values

Values that are determined within a regular expression should usually
be viewed as speculative, subject to cancellation if backtracking
occurs. This applies not only to the values captured by C<(...)> within
the regex, but also to values determined within closures embedded in
the regex. The scope of these values is rather strange, compared to
ordinary variables. They are dynamically scoped, but not like C<temp>
variables. A temporary variable is restored at the end of the current
block. A hypothetical variable keeps its value after the current block
exits, and in fact keeps that value clear to the end of its natural
lifetime if the regex succeeds (where the natural lifetime depends on
where it's declared). But if failure causes backtracking over where the
variable was set, then it is restored to its previous state. Perl 5
actually coerced the C<local> operator into supporting this behavior,
but that was a mistake. In Perl 6 C<temp> will keep consistent
semantics, and restore values on exit from the current block. A new
word, C<let>, will indicate the desire to set a variable to a
hypothetical value. (I was tempted to use "suppose", but "let" is
shorter, and tends to mean the same thing, at least to mathematicians.)

    my $x;
    / (\S*) { let $x = .pos } \s* foo /

After this pattern, C<$x> will be set to the ending position of
C<$1>--but only if the pattern succeeds. If it fails, C<$x> is restored
to C<undef> when the closure is backtracked. It's possible to do things
in a closure that the regex engine doesn't know how to backtrack, of
course, but a hypothetical value doesn't fall into that category. For
things that do fall into that category, perhaps we need to define a
C<BACK> block that is like C<UNDO>, but scoped to backtracking.

Sometimes we'll talk about declaring a hypothetical variable, but as
with C<temp> variables, we're not actually declaring the variable
itself, but the dynamic scope of its new value. In Perl 6, you can in
fact say:

    my $x = 0;
    ...
    {
        temp $x = 1;    # temporizes the lexical variable
        ...
    }
    # $x restored to 0

(This is primarily useful for dynamically scoping a file-scoped
lexical, which is slightly safer than temporizing a package variable
since nobody can see it outside the file.)

You may declare a hypothetical variable only when the topic is a regex
state. This is not as much of a hardship as it might seem. Suppose your
closure calls out to some other routine, and passes the regex state as
an argument, C<$rx_state>. It suffices to say:

    given $rx_state { let $x = .pos }

[Update: This restriction is no longer in effect.  Any variable
may be treated hypothetically, not just variables stored in C<$/>.
The capture variables in C<$/> are now syntactically distinguished
from ordinary variables.  Hypotheticality is orthogonal to that,
except insofar as C<$/> is somewhat hypothetical as a whole.]

As it happens, C<$1> and friends are all simply hypothetical variables.
When we say "hypothetical variable", we aren't speaking of where the
variable is stored, but rather how its contents are treated
dynamically. If a regex sets a hypothetical variable that was declared
with either C<my> or C<our> beforehand, then the regex modifies that
lexical or package variable, and C<let> is purely a run-time operation.

On the other hand, if the variable is not pre-declared, it's actually
stored in the regex state object. In this case, the C<let> also serves
to declare the variable as lexically scoped to the rest of the regex,
in addition to its run-time action. Such a variable is not directly
visible outside the regex, but you can get at it through the C<$0>
object (always presuming the match succeeded). For a regex variable
named C<$maybe>, its external name is C<$0._var_{'maybe'}>. The C<$0>
object can behave as a hash, so C<$0{maybe}> is the short way to say
that.

[Update: Nowadays the short name of that variable is C<< $<maybe> >>.
And the match variable is named C<$/> instead of C<$0>.]

All other variable names are stored with their sigil, so the external
name for C<@maybe> is C<$0{'@maybe'}>, and for C<%maybe> is
C<$0{'%maybe'}>.

[Update: None of these are stored with their sigil, which means
you can't store two different types under the same name unless you
explicitly bind to a name like C<< $<@maybe> >>.]

C<$1> is a special case--it's visible outside the regex, not because
it's predeclared, but because Perl already knows that the numbered
variable C<$1> is really stored as a subarray of the $0 object:
C<$0[1]>. The numbered variables are available only through the array,
not the hash.

[Update: the number variables are also available through the hash, so
C<$1> is the same as either C<< $/<1> >> or C<$/[0]>.]

Since C<$0> represents the state of the currently executing regex, you
can't use it within a rule to get at the result of a completed subrule.
When you successfully call a subrule named C<< <somerule> >>, the
regex state is automatically placed in a hypothetical variable named
C<$somerule>. (Rules accessed indirectly must be captured explicitly,
or they won't have a name by which you can get to them. More on that in
the next section.)

[Update: Any subrule's capture may be suppressed with by starting with
a question mark, so C<< <foo> >> captures but C<< <?foo> >> does not.]

As the current recursive regex executes, it automatically builds a tree
of hashes corresponding to all captured hypothetical variables. So from
outside the regex, you could get at the C<$1> of the subrule
C<< <somerule> >> by saying C<$0{somerule}[1]>.

[Update: These days that's C<$/{'somerule'}[0]> or C<< $<somerule><1> >>.]

=head2 Named Captures

Suppose you want to use a hypothetical variable to bind a name to a
capture:

    / (\S+) { let $x := $1 } /

A shorthand for that is:

    / $x:=(\S+) /

The parens are numbered independently of any name, so C<$x> is an alias
for C<$1>.

You may also use arrays to capture appropriately quantified patterns:

    / @x := (\S+ \s*)* /                # including space
    / @x := [ (\S+) \s* ]* /            # excluding space
    / @x := [ (\S+) (\s*) ]* /          # each element is [word, space]

Note that in general, naming square brackets doesn't cause the square
brackets to capture, but rather provides a destination for the parens
within the square brackets. Only parens and rules can capture. It's
illegal to name square brackets that don't capture something inside.

You can also capture to a hash:

    / %x := [ (\S+)\: \s* (.*) ]* /     # key/value pairs

After that match, C<$1> returns a list of keys, and C<$2> returns a
list of values. You can capture just the keys:

    / %x := [ (\S+) \s* ]* /            # just enter keys, values are undef

You can capture a closure's return value too:

    / $x := { "I'm in scalar context" } /
    / @x := { "I", "am", "in", "list", "context" } /
    / %x := { "I" => "am in hash context" } /

Note that these do not use parens. If you say:

    / $x := ({ code }) /

it would capture whatever text was traversed by the closure, but ignore
the closure's actual return value.

You can reorder paren groups by naming them with numeric variables:

    / $2:=(.*?), \h* $1:=(.*) /

If you use a numeric variable, the numeric variables will start
renumbering from that point, so subsequent captures can be of a known
number (which clobbers any previous association with that number). So
for instance you can reset the numbers for each alternative:

    / $1 := (.*?) (\:)  (.*) { process $1, $2, $3 }
    | $1 := (.*?) (=\>) (.*) { process $1, $2, $3 }
    | $1 := (.*?) (-\>) (.*) { process $1, $2, $3 }
    /

It's also possible to refer to captures relative to the current
location. C<$-1> refers to the immediately preceding capture (what used
to be known as C<$+>). C<$-2> refers to the one before that. If you use
anything above C<$-3> we'll come and take you away to the insane
asylum.

Subrules called via C<< <rule> >> also capture their result in
hypothetical variables. It's possible to name the results of any
C<< <...> >>, but grammar rules already have a name by default, so
you don't have to give them names unless you call the same rule more
than once. So, presuming you have grammar rules defining "key" and
"value", you can say:

    / <key> \: <value> { let %hash{$key} = $value } /

[Update: That is now written:

    / <key> \: <value> { let %hash{$<key>} = $<value> } /

]

Of course, in a typical grammar the typical rule may not return a
string, but a reference to an anonymous object representing a node of
the parse tree. But that depends on what the subrule decides to
capture. If the only thing captured in the subrule is a single string,
that's what you get. (If nothing is captured, you get the entire
match.)

Any capture that will capture multiple items will, if asked to put it
into a scalar variable, produce an anonymous list automatically. This
should rarely be a surprise, since it's obvious by inspection whether
you've quantified the capture or not. So if you say any of:

    / $x := <word>*/
    / $x := <word>+/
    / $x := <word><1,3>/

[Update: That last should use C<**{1..3}> instead.]

then you've "pluralized" the naming, and you can expect to get some
number of values in C<$x> as an anonymous list. However, the C<?>
quantifier specifically doesn't pluralize. If you say:

    / $x := <word>?/

then C<$x> will either be the result of the subrule or C<undef>.

You can name the results of a zero-width assertion, but you'd typically
only get the null string out of it. This can still be useful, since it
contrasts with the undefined value you'd have if the assertion fails.
(It is possible with an explicit capture to return a non-zero-width
string from a zero-width assertion, however.)

=head2 Variable Scoping

When you refer to a variable C<@foo> as an rvalue in a regex, it
searches for an existing variable in the following places:

=over

=item 1. We first look to see if the variable is already declared
lexically with either C<my @foo> or C<our @foo>. If so declared, we're
done.

=item 2. Next we look for C<@foo> in the current regex's name table.
The name of the variable is really C<$0{'@foo'}>.

[Update: These C<$/> variables now have names like C<< @<foo> >>, so skip
this step.]

=item 3. If the regex belongs to a grammar, we next look for C<@foo> in
the grammar object. If there, its real name is C<@.foo>, or some such.
(It might be objected that the grammar object is not yet constructed
when the regex is compiled. After all, the regex is probably being
passed to the grammar object's constructor. But I think if such a
variable is declared as an object attribute we know that there I<will>
be such a variable/accessor later when we have finished constructing,
and that seems like enough info to know how to compile the regex.)

[Update: I think we can skip this one too--just import the array if
you need it, or call it via a real rule accessor.]

=item 4. Next we look for C<@foo> as a declared core global variable
C<@*foo>.

=item 5. Finally, if "strict vars" is not in effect, we assume that
C<@foo> is stored in the current package. Otherwise it's a stricture
error.

=back

=head2 Variable Interpretation

As we mentioned earlier, bare scalars match their contents literally.
(Use C<< <$var> >> instead to match a regex defined in C<$var>.)
Subscripted arrays and hashes behave just like a scalar as long as the
subscripts aren't slices.

If you use a bare array (unsubscripted), it will match if I<any>
element of the array matches literally at that point. (A slice of an
array or hash also behaves this way.) If you say

    @array = ("^", "$", ".");
    / @array /

it's as if you said

    / \^ | \$ | \. /

But if you you slice it like this:

    / @array[0..1] /

it won't match the dot.

If you want the array to be considered as a set of regex alternatives,
enclose in angles:

    @array = ("^foo$", "^bar$", "^baz$");
    / <@array> /

Bare hashes in a regex provide a sophisticated match-via-lookup
mechanism. Bare hashes are matched as follows:

=over

=item 1. Match a key at the current point in the string.

=over

=item 1a. If the hash has its C<keymatch> property set to some regex,
use that regex to match the key.

=item 1b. Otherwise, use C</\w+:/> to match the key.

=back

=item 2. If a key isn't found at the current position in the string,
the match fails.

=item 3. Otherwise, get the value in the hash corresponding to the
matched key.

=item 4. If the is no entry for that key, the match fails.

=item 5. If the hash doesn't have a C<valuematch> property, the match
succeeds immediately.

=item 6. Otherwise use the hash's C<valuematch> property (typically
itself a regex) to extract the value at the current point in the
string.

=item 7. If no value can be extracted, matching of the hash fails.

=item 8. If the extracted value string is C<eq> to the key's actual
value, matching of the original hash immediately succeeds.

=item 9. Otherwise, matching of the original hash fails.

=back

So matching a bare hash is equivalent to:

    rule {
        $key := <{ %hash.prop{keymatch} // /\w+:/ }>    # find key
        <( exists %hash{$key} )>                        # if exists
        [ <( not defined %hash.prop{valuematch} )> ::   # done?
            <null>                                      # succeed
        |                                               # else
            $val := <%hash.prop{valuematch}>            # find value
                <( $val eq %hash{$val} )>               # assert eq
        ]
    }

A typical C<valuematch> might look like:

    rule {
        \s* =\> \s*             # match => 
        $q:=(<["']>)            # match initial quote 
        $0:=( [ \\. | . ]*? )   # return matched value
        $q                      # match trailing quote 
    }

In essence, the presence or absence of the C<valuematch> property
controls whether the hash tries to match only keys, or both keys and
values.

[Update: The above is all completely bogus.  A hash's set of
keys are simply compiled into a longest-first token match table.
The corresponding value contains the closure or rule to fire off when
you match, or a boolean to make the key match merely succeed.  See S5.]

A hash may be used inside angles as well. In that case, it finds the
key by the same method (steps 1 and 2 above), but always treats the
corresponding hash value as a regex (regardless of any properties the
hash might have). The parse then continues according to the rule found
in the hash. For example, we could parse a set of control structures
with:

    rule { <%controls> }

The C<%controls> hash can have keys like "C<if>" and "C<while>" in it.
The corresponding entry says how to parse the rest of an C<if> or a
C<while> statement. For example:

        %controls = ( 
            if     => / <condition>      <closure> /,
            unless => / <condition>      <closure> /,
            while  => / <condition>      <closure> /,
            until  => / <condition>      <closure> /,
            for    => / <list_expr>      <closure> /,
            loop   => / <loop_controls>? <closure> /,
        );

So saying:

    <%controls>

is really much as if we'd said:

    [ if     \b <%controls{if}>
    | unless \b <%controls{unless}> 
    | while  \b <%controls{while}>
    | until  \b <%controls{until}> 
    | for    \b <%controls{for}> 
    | loop   \b <%controls{loop}>
    ]

Only it actually works more like

    / $k=<{ %controls.prop{keymatch} // /\w+:/ }> <%controls{$k}> /

[Update: This is also bogus.  The keys are treated as a set of rules with
longest-first semantics for any leading literal components.  See S5.]

Note that in Perl 6 it's perfectly valid to use C<//> inside an
expression embedded in a regex delimited by slashes. That's because a
regex is no longer considered a string, so we don't have to find the
end of it before we parse it. Since we can parse it in one pass, the
expression parser can handle the C<//> when it gets to it without
worrying about the outer slash, and the final slash is recognized as
the terminator by the regex parser without having to worry about
anything the expression parser saw.

A bare subroutine call may be used in a regex, provided it starts with
C<&> and uses parentheses around the arguments. The return value of the
subroutine is matched literally. The subroutine may have side effects,
and may throw an exception to fail.

=head2 Defining Your Own Rules

Suppose your name is Hugo and you don't like to use C<!> to negate an
assertion. You can define your own assertion like this:

    my rule not (str $rx) { <!<{"<$rx>"}>> }            # define Hugo not
    / <not [a-z]> /     # same as <![a-z]>

That rule would be lexically scoped because of the C<my>. If you think
it looks like a C<sub> declaration, you're right. In fact, it's
possible you could even declare it anonymously like a closure:

    my $not = rule (str $rx) { <!<{"<$rx>"}>> };
    / <$not tonight dear> /

But maybe you don't want it lexically scoped because you're writing a
grammar for general use:

    grammar HugoGrammar {
         rule not ($rx) { <!$rx> }
         rule identifier { <not before \d> \w+ }
         rule \j { \c[LF] }
         rule parse { ^ <identifier> \j $ }
    }
    HugoGrammar.parse($line);

In this case a rule is simply a method in a grammar class, and a
grammar class is any class derived implicitly or explicitly from the
universal RULE grammar class. The built-in regex assertions like
C<< <before \w> >> are really just calls to methods in the RULE
class. The namespace of a grammar is simply the method namespace of the
current class, which is the class's methods plus all inherited methods.

[Update: Nowadays it's the C<Rule> class.]

In addition to normal subrules, we allow some funny looking method
names like:

    rule :a { ... }
    rule \a { ... }

[Update: These are now handled as macro-like syntactic categories.  See S5.]

Modules that mutate Perl's grammar on the fly can do so by deriving an
anonymous grammar class from the default Perl6Grammar, and installing
extra rules on the fly. The current regex state then continues parsing
the rest of the lexical scope using some rule from the new rule set.
Subsequent grammatical mutations will be derived from the current
anonymous grammar unless you switch explicitly to an entirely different
grammar.

Since we're writing grammar rules as if they were methods, we have
access to the full syntax of method declaration, including formal
parameter lists and compile-time properties. So we can easily annotate
rules with pragmatic information such as operator precedence levels
when you don't want to write a strictly recursive-descent parser, for
instance. (And we don't want to, for Perl.)

=head1 Accepted RFCs

=head2 RFC 072: Variable-length lookbehind.

This seems good to me. It's just a SMOP to reverse the ordering of
nodes in the syntax tree, and I think we can pretty well determine when
it's impossible to reverse the tree. The operation of a reversed syntax
tree will not be totally transparent, however, so it will be necessary
to document that quantifiers will actually be working right-to-left
rather than left-to-right. (It's probably also a good idea to document
that many syntactic constructs can't actually be reliably recognized in
reverse. An attempt to do so probably means you needed to do a
lookahead earlier, rather than a lookbehind later.)

The syntax of lookbehind uses the new assertion syntax:

    <after ...>         # positive lookbehind
    <!after ...>        # negative lookbehind

Yes, the C<pos()> function could return multiple values in list
context, but I think it's more reasonable for the individual captured
elements to know where their positions are. The C<pos> function is
really just a special case of a more general data structure contained
in the regex result object from the last successful match. In which
case, maybe it really needs to have a better name than C<pos>. Maybe
C<$0> or something. Then we get C<$0.beg> and C<$0.end>, C<$1.beg>, and
C<$1.end>, etc. Since C<@$0> returns a list of captures, you can do
C<@$0^.beg> and C<@$0^.end> if you want a list of beginnings and
endings. Did I mention that the magical C<@+> and C<@-> arrays are
gonna be real dead? Never could remember which one was which anyway...

[Update: C<$0> still represents the entire matched string, but the
match object is now C<$/>, and a list of all beginnings is returned
by the hyperoperator C<$/».beg>.  But note that string positions are
not necessarily integers in Perl 6.  They are tagged with the units
of the string, so that you can't inadvertently mix byte, codepoint,
or grapheme offsets.]

=head2 RFC 093: Regex: Support for incremental pattern matching

I don't think this proposal is powerful enough. "Infinite" strings are
a more powerful concept. But I don't think infinite strings are
powerful enough either!

We're certainly going to have "infinite" arrays for which missing
elements are defined by a generator (where the action could be as
simple as reading more data from some other source). We could do the
same thing for strings directly, or we could define strings that are
implemented underneath via arrays (of strings or of stringifiable
objects), and achieve infinitude that way. This latter approach has the
benefit that the array element boundaries could be meaningful as
zero-width boundaries between, say, tokens in a token stream. We're
thinking that C<< <,> >> could match such a boundary.

But beyond that, such arrays-as-strings could allow us to associate
hidden metadata with the tokens, if the abstract string is constructed
from a list of objects, or a list of strings with properties. This is
typically how a parser would receive data from a lexical analyzer. It's
the parser's job to transform the linear stream of objects into a parse
tree of objects.

Matching against such boundaries or metadata would not be possible
unless either the regex engine is aware that it is matching against an
array, or the string emulation provides visibility through the abstract
string into the underlying array. The latter may be preferable, since
(by the rules of the C<=~> matrix discussed in Apocalypse 4) C<@array
=~ /regex/> is currently interpreted as matching against each element
of the array individually rather than sequentially, and there are other
uses for a string that's really an array. In fact, C<@array =~ /regex/>
could conceivably be matching against a set of infinite strings in
parallel, though that seems a bit scary.

[Update: The C<=~> operator is renamed C<~~>, and it doesn't automatically
"any-fy" an array anymore, so we could pretty easily make it work over an
entire array as if it were a string.]

Even if we don't care about the boundaries between array elements, this
approach gives us the ability to read a file in chunks and not worry
that the pattern won't match because it happens to span a boundary.

It might be objected that matching against a subroutine rather than an
infinite string or array has the benefit of not promising to keep
around the entire string or array in memory. But this is not really a
feature, since in general a regex can potentially backtrack all the way
to the beginning of the string. And there's nothing to say that the
front of the infinite string or array has to stay around anyway.
Whether to throw away the head of a string or array should really
depend on the programmer, and I don't think there's a more intuitive
way to manage that than to simply let the programmer whack off the
front of the string or array using operators like C<substr> or
C<splice>, or the new C<< <cut> >> assertion.

Indeed, the very existence of the string/array precludes the caching
problem that RFC 316 complains about.

The question remains how to declare such a string/array. If we decided
to do a magical name identification, we could conceivably declare

    my $@array;

and then both C<$array> and C<@array> refer to the same object, but
treated as a string when you say C<$array> and as an array when you say
C<@array)>. One is tempted to set up the input routine by saying

    my $@array is from { <$input> };

Additional lines (or chunks) would then come from the
C<< <$input> >> iterator.

But really, the infinite nature of the array is a feature of the
underlying object, not the variable. After all, we want to be able to
say

    @array := 1..Inf;

even with an ordinary array.

So we could even make this work:

    my $@array := <$input>;

But I think we need to break the aliasing down, which will give us more
flexibility at the expense of more verbiage:

    my @array := <$input>;              # @array now bound to iterator
    my $array is ArrayString(@array);   # an ordinary tie

That would let us do cool and/or sick things like this:

    my @lines := <$article>;
    my $_ is ArrayString(@lines);
    s/^ .*? \n<2,> //;  # delete header from $_ AND @lines!
    for @lines { ... }  # process remaining lines

The C<for> loop potentially runs forever, since C<@lines> is implicitly
extended from an iterator. The array is automatically extended on the
end, but it's not automatically shifted on the front. So if you really
did want the loop to run forever without exhausting memory, you'd need
to say something like:

    substr($_, 0, $_.pos, "");

The same effect can be effected within a regex by asserting
C<< <cut> >>, which makes the current position the new string
beginning. (If you backtrack over C<< <cut> >>, the entire match
will fail.)

[Update: Now we can just say C<<@lines ~~ s/^.*? \n**{2...} //>>.]

=head2 RFC 110: counting matches

I think we can avoid using any options if we make a pattern count
matches when used in a numeric context. If in doubt, make it explicit:

    $count = +/foo/;

If it turns out we do need an option, it'll probably be C<:n>.

=head2 RFC 112: Assignment within a regex

This RFC is basically covered by the C<$foo:=(...)> notation, plus
variations. The RFC claims that such assignments are not done till the
end, except that they are done ahead of closures. I'd rather state it
the other way around: it always appears that the current hypothetical
binding is assigned if you check, but as long as the optimizer can
determine that you aren't looking, it doesn't have to keep up
appearances. Contrariwise, if C<$foo> is just a fancy way of saying
C<$1>, there may in fact be no more overhead in maintaining C<$foo>
than C<$1>. Either is really just pointing into a table of offsets into
the string. That's assuming we get the scoping right on hypothetical
variables.

Some excerpts from the RFC:

=over

=item I<The camel and the docs include this example:>

       if (/Time: (..):(..):(..)/) {
            $hours = $1;
            $minutes = $2;
            $seconds = $3;
        }

I<This then becomes:>

      /Time: (?$hours=..):(?$minutes=..):(?$seconds=..)/

=back

Now that looks like this:

      /Time\: $hours:=(..) \: $minutes:=(..) \: $seconds:=(..)/

=over

=item I<It may be appropriate for any assignments made before a code
callout to be localized so they can unrolled should the expression
finally fail.>

=back

Rather than localized (or temporized), they are hypothesized.

=over

=item I<The first versions of this RFC did not allow for backrefs. I
now think this was a shortcoming. It can be done with C<(??{quotemeta
$foo})>, but I find this clumsy, a better way of using a named back ref
might be C<(?\$foo)>.>

=back

Backrefs are now unified with hypothetical variables, so the issue
doesn't arise. Just use C<$foo>.

[Update: Now C<< $<foo> >>, short for C<$/{'foo'}>.]

=over

=item I<Using this method for capturing wanted content, it might be
desirable to stop ordinary brackets capturing, and needing to use
C<(?:...)>. I therefore suggest that as an enhancement to regexes that
/b (bracket?) ordinary brackets just group, without capture - in effect
they all behave as C<(?:...)>.>

=back

There's no need for a C</b> now that we have C<[...]> for non-capturing
brackets.

=head2 RFC 144: Behavior of empty regex should be simple

I agree, the behavior should be simple. However, rather than always
matching, I propose to make it an error to actually have a null
pattern, or a null choice in a list of alternatives. Use an explicit
C<< <null> >> if that's what you mean. (It's not a problem if
C<$foo> is null in C</$foo/>, since variables are now managed by the
regex engine and not by interpolation.)

=head2 RFC 150: Extend regex syntax to provide for return of a hash of
matched subpatterns

The C<$foo:=(...)> notation essentially covers that case. One can say:

    / %hash{foo}:=(...) %hash{bar}:=(...) /

Fancier things can be done with closures.

=head2 RFC 156: Replace first match function (C<?...?>) with a flag to
the match command.

Having a C<:f> modifier seems like a reasonable way to do it:

    m:f/.../

Though it's vaguely possible we should be having a set of verbs that
parse like C<split>:

    split /.../
    count /.../
    first /.../

It's not clear whether those are actually methods, and if so, on which
object, the string or the regex. In any event, I don't think we have to
nail that down quite yet. I'm accepting the basic premise of this RFC
that the C<?...?> construct is going away, one way or another.

At the moment, it looks like this option is spelled C<:once>.

=head2 RFC 165: Allow Variables in tr///

If interpolation of patterns by default is wrong, I think extending the
C<tr///> interface via scalar interpolation is doubly wrong. Run-time
generated transliterations should be based on mappings that aren't so
position dependent. That is, rather than specifying it as two long
lists:

    abc12xyz => ABC34XYZ

we specify something more like this:

    abc => ABC
    12  => 34
    xyz => XYZ

That looks more like a list of pairs of scalars than a pair of scalars.
In fact, internally, it's done like a funny parallel substitution:

    s:e(/a/A/,
        /b/B/,
        /c/C/,
        /1/3/,
        /2/4/,
        /x/X/,
        /y/Y/,
        /z/Z/)

In any event, it's more like C<tr/@foo/@bar/> than C<tr/$foo/$bar/>.
But then, why stick with the fake string notation? Why not just say
C<tr(@foo,@bar)> if that's what we mean? Then we're not limited to
character substitutions:

    $string.tr [ " "     , "<"   , ">"   , "\n"   ],
               [ "&nbsp;", "&lt;", "&gt;", "<br>" ];

Or how about C<tr(%trans)>?

    %upper = {
        "a-z" => "A-Z",
    }
    $string.tr %upper;

or just pair lists of some sort:

    $string.tr("a-c" => "x-z",
               "1-2" => "3-4",
               "A-C" => "X-Z",
              );
    @trans = [
        "a-z" => "A-Z",
        @tr_danish,
    ];
    $string.tr(@trans)

=head2 RFC 166: Alternative lists and quoting of things

Alternative lists of literals are included simply by mentioning the
array:

    /@names/

Alternative lists of subrules are included with:

    /<@names>/

There's no longer any need for quoting constructs because variables
match as literals by default. You have to use angle brackets to get
interpretation of a string as a subrule. (But it's still preferable to
precompile your regexen.)

=head2 RFC 191: smart container slicing

As proposed, this might prevent us from using a regex object as a key
to a hash. However, with some tweaking, it'll fit in with how slicing
is done in Perl 6.

Perl 6 will DWIM subscripts based on their appearance. Obviously,

    %hash{"foo"}

has a single subscript. And just as obviously,

    %hash{"a" .. "z"}

has 26 subscripts or so. In the absence of any scalar guidance, a
subscript will be interpreted in list context. So

    %hash{ @array }

will automatically slice on the list of keys in the array. Any function
will be called in a list context by default, giving it the opportunity
to return multiple values. Perl 6 subscripts are naturally biased
toward slicing. To unbias it, here are some of the specifically
recognized subscripts:

    %hash{"foo"}        # scalar literal
    %hash{bar}          # scalar literal

[Update: Now C<< %hash<bar> >> instead.]

    %hash{1}            # scalar literal
    %hash{$x}           # scalar variable
    %hash{\$x}          # scalar reference
    %hash{["a", "b"]}   # array reference
    %hash{{"a" => "b"}} # hash reference
    %hash{ "a" => "b" } # pair reference
    %hash{ /pat/ }      # rule reference
    %hash{ _ expr }     # force expr to return a single string

[Update: Now C<~> instead of C<_>.  Also, active slicing with
closures is done with a C<.slice> method on either arrays or hashes.
Ordinary subscripts always assume integer slices for arrays and string
slices for hashes (by default).  The optimizer is allowed to assume
that in C<@foo[@bar]>, the @bar array returns a list of integers.]

    %hash{ + expr }     # force expr to return a single number

Boolean expressions and closures look like singular values but cause a
match against all possible values of the subscript.

    %hash{ ?1 }         # select all subscripts
    %hash{ ?/pat/ }     # select subscripts for which pat matches
    %hash{ $_ =~ /pat/ }# select subscripts for which pat matches
    %hash{ $_ ge "a" }  # select lowercase keys (assuming ASCII)
    %hash{ .ge "a" }    # same thing, maybe
    %hash{ { expr } }   # select subscripts for which closure returns true

Multiple slice subscripts are separated by semicolons, so that you can
use commas within each slice subscript for list building. This is more
important for multi-dimensional arrays:

    my @array is dim(9,9,9) = cubic();
    @3d_slice = @array[ @x; @y; @z ];
    @3d_slice = @array[ 0,1,3,8 ; 0,1,3,8; ?1 ];
    @3d_slice = @array[ 0..9 ; 0..9:-1; ?test($_) ];
    @3d_slice = @array[ !($_ % 2) ; 0..9:3; ?test($_) ];

[Update: C<dim> is now spelled C<shape>.  See S9.]

=head2 RFC 274: Generalized Additions to Regexs

This proposal has significant early/late binding issues. A definition
that forces run-time overhead is not as useful as it might be. On the
other hand, a pure compile-time mechanism is not as general as it might
be--but a compile-time mechanism can always compile in a run-time
mechanism if it chooses to defer evaluation.

So it seems like this is a good place for syntactic warpage of some
sort or other. That would make it possible to do both compile-time and
run-time bindings. We'll be using the C<< <...> >> notation for our
extensible syntax, and the grammar rules for parsing that particular
part of Perl syntax will be just as easy to tweak as any other Perl
grammar rule.

That being said, the very fact that we can associate a grammar with the
regex means that it's easy to define any instance of C<< <word> >>
to mean whatever you want it to. (In a sense, these subrules are the
very callbacks that the RFC proposes.) These subrules can be bound
either at Perl compile time or at Perl run time. They can be defined to
take a string, regex, or Perl expression as an argument. The latter two
cases are efficient because they come in as a regex or code reference
respectively.

=over

=item I<Following on, if (?{...}) etc code is evaluated in forward
match, it would be a good idea to likewise support some code block that
is ignored on a forward match but is executed when the code is unwound
due to backtracking.>

=back

Yes, though hypothetical values take some of the pressure off for this.
But if a closure contained a BACK block, it could be automatically
fired off on backtracking. As with LAST et al., I suppose there's a
corresponding C<back> property on variables. In a sense, saying

    let $var = $newval

is much like saying

    our $var is back { .set($oldval) } = $newval

except that C<$var> may well be stored in the regex state object rather
than in a global symbol table.

=head2 RFC 276: Localising Paren Counts in qr()s.

I agree totally. As for the problem of pulling captures out of a
subrule, it's up to the subrule to determine what it "returns". We
could make some intelligent defaults, though different kinds of rules
might want different defaults. One approach might be to say that if
there is a single capture, that is returned as the result. If there is
no capture, it's as if the entire subpattern were captured. If there
are multiple captures, they are returned as an anonymous list. So C<$1>
from such a subrule might come through like this:

    / $sub:=<subrule> { print $sub[1] } /

or just:

    / <subrule> { print $subrule[1] } /

But named captures and named rules intrude on this idyllic picture.
You'd also like a default anonymous hash value returned that is keyed
by all the named captures or rules. The question is whether that forces
numbered captures to come through the hash interface. Or maybe that's
just always the case, so to get at C<$1> of a subrule, you'd say:

    / $sub:=<subrule> { print $sub{'1'} } /

But there are reasons for wanting to treat the result object as an
array, so that

    / $sub:=<subrule> { process(@$sub) } /

processes all the numbered captures from the subrule. So I think the
return object behaves either like a hash or an array as appropriate.
(Note that such an array might be declared to have an origin at 1
rather than 0.)

[Update: We're steering clear of 1-based arrays for now.]

=head2 RFC 317: Access to optimisation information for regular
expressions

Seems like a no-brainer. All such information is likely to be available
to Perl anyway, given that we'd like to do the parser, optimizers, and
code generators in Perl if at all possible.

=head2 RFC 331: Consolidate the C<$1> and C<\1> notations

I like the title of this RFC. It fits in with the new C<my> policy of
immediate introduction. However, there are certain difficulties with
the proposed implementation. The statement-by-statement setting of the
C<@/> array looks pretty ugly to me. I'd rather have a consistent view
of hypothetical variables that can live on outside the regex in
question without regard to statement boundaries. In the rare event that
someone needs to refer to C<$1> (or anything else) from a prior regex,
a temporary variable should be used.

=head2 RFC 332: Regex: Make /$/ equivalent to /\z/ under the '/s'
modifier

Another RFC that is accepted in principle, but that doesn't go far
enough. The C</s> modifier is going away, along with C</m>. A C<$> will
always mean end-of-string, and C<$$> will match at the end of any line.
(The current process id is now C<$*PID>, by the way, so there's no
conflict there. But how often to you want to write a pattern to match
the current process id anyway?)

=head2 RFC 348: Regex assertions in plain Perl code

This RFC makes some good points, though the code assertion syntax will
be:

    <( code )>

The RFC also makes a case for getting rid of the special behavior of
C<local> in Perl 5, which treated C<local> differently within a regex.
However, something very like the C<local> behavior will still be needed
for making hypotheses, though the RFC is correct that it's not needed
in the typical code assertion, In Perl 6, localization is done with
C<temp>, and it will not do the hypothetical variable hack that Perl 5
did. Instead there will be an explicit lvalue modifier, C<let>, which
specifically requests a variable's value to be scoped to the success of
the current point in the regex. These hypothetical variables actually
have much broader use than this RFC suggests.

Perl 5's hardwired use of C<$^R> just translates to an appropriately
named hypothetical variable in Perl 6.

=head2 RFC 360: Allow multiply matched groups in regexes to return a
listref of all matches

I think that parens that can potentially match multiple times will
automatically produce a list rather than matching the final one. It's
not as if we can't tell whether something's inside a quantifier...

Here's the RFC's proposed solution:

    while ($text =~ /name:\s*(.*?)\n\s*
                    children:\s*(?:(?@\S+)[, ]*)*\n\s*
                    favorite\ colors:\s*(?:(?@\S+)[, ]*)*\n/sigx) {
        # now we have:
        #  $1 = "John Abajace";
        #  $2 = ["Tom", "Dick", "Harry"]
        #  $3 = ["red", "green", "blue"]
    }

Apart from the change in behavior of C<(...)> within a quantifier, I
have the urge to rewrite this example for several reasons:

=over

=item *

    The C</x> and C</s> flags no longer exist.

=item *

    The C</i> and C</g> flags must be pulled out to the front for visibility.
    (And the C</g> flag is renamed C<:e>).

=item *

    There's now a C<\h> for horizontal whitespace, and C<\H> for the negation
    of that.  (Not that RFC is incorrect to use C<\s>.)

=item *

    The negation of C<\n> is now C<\N>.

=item *

    The C<:> character is now a metacharacter, and so must be backslashed.

=item *

    Character classes are now represented with C<< <[...]> >>.

=item *

    Grouping is now represented with C<[...]>.

=back

With these changes, and making better use of whitespace, the sample
regex ends up looking like this:

    for ($text =~ m:ie[
                            name             \: \h*   (\N*?)            \n
                        \h* children         \: \h* [ (\S+) <[,\h]>* ]* \n
                        \h* favorite\ colors \: \h* [ (\S+) <[,\h]>* ]* \n
                      ]
          )
    {
             # now we have:
             #  $1 = "John Abajace";
             #  $2 = ["Tom", "Dick", "Harry"]
             #  $3 = ["red", "green", "blue"]
    }

I think in the long run people will find this more readable once
they're used to it. Certainly tabularizing the parallelisms will make
any typing errors stand out.

[Update: The C<:ie> is now written C<:i:g>.]

=head2 RFC 361: Simplifying C<split()>

The RFC makes five suggestions. I'll consider them one by one.

=over

=item I<The first argument to split is currently interpreted as a
regexp, regardless of whether or not it actually is one. (Yes, 
C<split '.', $foo> doesn't split on dot -- it's currently the same an 
C<split /./, $foo>.) I suggest that split be changed to treat only 
regexps as regexps, and everything else as literals.>

=back

Fine, I think. If the first argument to C<split> is untyped, it should
parse correctly, either evaluating a quoted string immediately or
deferring interpretation of a regex. One could even do something like
split on the first delimiter matched by another pattern:

    split _/(,|;)/;

That would split on either all commas or all semicolons, depending on
which it found first in the string. The _ forces the regex to return a
string, which is whatever was captured by the parens in this case.

[Update: C<_> is now C<~>.]

=over

=item I<Empty trailing fields are currently suppressed (although a -1
as the third argument disables this). I suggest that empty trailing
fields be retained by default.>

=back

Probably okay, though we need a way to translate old code. It was
originally done this way because split on whitespace would typically
return an extra field after the newline. But most newlines will be
prechomped in Perl 6.

=over

=item I<When not in list context, split currently splits into C<@_>. I
suggest that this side-effect be removed.>

=back

Fine. It's easy enough to translate to an explicit assignment.

=over

=item I<C<split ?pat?> in any context currently splits into C<@_>. I
suggest that this side-effect be removed.>

=back

Fine. I don't think anyone uses that.

=over

=item I<C<split ' '> (but not C<split / />) currently splits on
whitespace, but also removes leading empty fields. I suggest that this
irregularity be removed.>

=back

The question is, what to replace it with, since it's a very handy
construct. We could use a different conventional pattern:

    @array = split /<ws>/, $string;

Or we could say that it's now a split on whitespace only if the split
argument is unspecified. That wouldn't work very well with the old
syntax, where we often have to supply the second argument. But given
that the C<=~> operator now serves as a topicalizer for any term, we
could translate:

    @array = split ' ', $string;

to this:

    @array = $string =~ split;

Oddly, this probably also works:

    $string =~ (@array = split);

or maybe even this:

    @array = split given $string;

But I think I like the OO notation better here anyway:

    @array = $string.split;

In fact, split may not be a function at all. The default split might
just be a string method and use unary dot:

    @array = .split;

We still have the third argument to deal with, but that's likely to be
specified like this:

    @array = $string.split(limit => 3);

We could conceivably make a different method for word splitting, much
like REXX does:

    @array = .words;

Then a limit could be the first argument:

    @array = .words(3);

But there almost doesn't need to be such a method, since

    @array = m/ [ (\S*) \s* ]* /;

will do the right thing. Admittedly, a C<.words> method would be much
more readable...

Fortunately, C<split> is a function, so I can put off that decision
till Apocalypse 29. C<:-)>

[Update: At the moment I think there's a C<.words> multimethod on strings.]

=head1 Rejected RFCs

=head2 RFC 135: Require explicit m on matches, even with ?? and // as
delimiters.

Squish that gnat... C<:-)>

A decent Perl parser is still going to have to keep track of whether a
term or an operator is expected. And while we're simplifying the
grammar in many ways, it's also the case that we're letting users
install their own grammar rules to perform syntactic warpage. Besides,
people I<like> to write patterns with C</.../>. So rather than
impoverishing Perl's syntax artificially, let's make the standard
parser more accessible by writing it all in Perl 6 regexes.

=head2 RFC 145: Brace-matching for Perl Regular Expressions

Good problem, not-so-good solution from a complexity point of view. I'd
like to leverage existing character class and backref notations maybe.
If there were simply some way to tell a backref to invert any match
characters, that might do it. Or maybe reverse them when you remember
them, and leave the backref ignorant? (Downside is nested brackets
would probably need recursive patterns.)

Recursion might be advisable anyway--you can't really pick up the
arguments to a function, for instance, without also handling things
like quoted strings, which may have different bracketing rules than
outside of strings. Certainly matching C<\"> would be dependent on
whether you're inside or outside of a string. Given that recursion is
often necessary, I'm not sure making this construct recurse itself is
all that useful.

Along the lines of how C<tr///> works (or ought to work), I think it'd
be more generally useful to have character remapping facility within a
backref generator:

    (
     <[ \( \[ \{ \< ] =>
      [ \) \] \} \> ]> )

That might match a left bracket of some sort but return the
corresponding right bracket as C<$1>. But maybe we should just use an
"existing" mechanism to translate strings:

    my %closing = {
        '[' => ']',
        '(' => ')',
        '{' => '}',
        '<' => '>',
    };
    rule balanced {
        <![\[\(\{\<\]\)\}\>]>*  # any non-brackets
        [                       # followed by either
            $                   #   end of string
        |                       # or
            $b := <[[({<]>      #   an opening bracket
            <self>              #   containing a balanced expr
            %closing{$b}        #   followed by corresponding close bracket
            <self>              #   followed by a balanced expr
        ]
     }

=head2 RFC 164: Replace =~, !~, m//, s///, and tr// with match(),
subst(), and trade()

All operators will have a way to name them, which means it's possible
to alias them to any other name. Rearranging the formal order of
parameters would be a little harder, however. We need inlining to do
that efficiently. Still, now that C<//> doesn't evaluate in a typeless
context, it's relatively straightforward to define a subroutine or
method that does

    subst $string, /foo/, {"bar"}

in whatever order you like.

[Update: We're defining standard C<.match>, C<.subst>, and C<.trans>
methods for strings as alternatives to the quoted syntaxes.]

=head2 RFC 197: Numeric Value Ranges In Regular Expressions

If we go down this road, eventually we reinvent all of Perl syntax in
regular expressions. Not that I'm against TMTOWTDI, but I'd rather have
a better way to run Perl code from within a regex and have it "succeed"
or "fail", and maybe better ways to test ranges from Perl code.
Anything beyond that could be done with syntactic warpage.

In any event, overloading C<()> and C<[]> for this would be mentally
treacherous, not to mention completely opaque to non-mathematicians.
We'll stick with the standard boolean assertion:

    / (\d+) <( $1 =~ 1..10 )> /

Interestingly, that can also be written:

    / <( _/\d+/ =~ 1..10 )> /

[Update: That'd be C<< <( ~/\d+/ ~~ 1..10 )> >> these days.]

=head2 RFC 198: Boolean Regexes

Again, I'm not much in favor of inventing new regex syntax that
duplicates ordinary Perl syntax. I think we need richer ways of
interconnecting related regexes via ordinary Perl syntax. Certainly it
helps to have an easy way to specify a Perl assertion:

    / (\w+) <( %count{$1} > 3 )> /

But there's something to be said for forcing submatch assertions to be
defined externally to the current regex, much like we discourage inline
code where subroutine calls are in order.

So anyway, I think most of the submatches like onion rings should be
handled simply by searching on captured strings within a closure.
Booleans can be put into closures as well, but the new C<::> operator
makes it pretty easy to AND and OR assertions together in a more
regexly fashion without reinventing the wheel.

As proposed, there will be a "fail" token, but it's spelled
C<< <fail> >>, not C<\F>. And the "true" token is spelled
C<< <null> >>. C<:-)>

=head2 RFC 261: Pattern matching on perl values

This reminds me a bit of unification in Prolog. It's not explained very
well here, and I'm wondering if it will be too hard to explain in
general. I think this is probably too powerful a concept for the
typical Perl programmer, who is lucky to understand simple lvalues that
always do what they're told.

This sort of matching can probably be provided as syntactic warpage,
though I'm not sure if that prevents useful optimizations. Anyway, this
sort of thing is unlikely to make it into the Perl 6 core unless it
generalizes usefully to function argument lists, and it may be too
powerful for there too. For that purpose it would resemble a form of
overloading, but with the "types" specified by keys. I suspect real
types are more useful.

=head2 RFC 308: Ban Perl hooks into regexes

We must be able to call back into Perl code if we want to write parsers
conveniently in Perl. Think of how yacc works. Certainly the way that
Perl 5 does it is ugly, I'll admit. We can beautify that.

But the whole point of Perl is to have all the most useful "Krakken
tentacles". And I don't really care if it makes it hard to put the Perl
regex engine into some other language. C<:-)>

=head2 RFC 316: Regex modifier for support of chunk processing and
prefix matching

Infinite strings (via infinite arrays) seem like a more useful concept.
It would be easy for the extension subroutine to fail and produce the
results desired in this RFC, but without the necessity of the extra
syntax specified by the RFC. A match naturally fails when it gets to
the end of its string without finishing the pattern. Incremental
matching can also easily be done via infinite strings, and the user
interface can be a simple as we like, as long as extension rule is
somehow associated with the string in question.

I think C<pos()> is rather too low-level a concept for general use.
Certainly it needs to be there, but I think we need some way of
implying that one regex is a continuation of a previous one, but within
some higher-level syntactic construct, so that it's easy to write
parsers without invoking C<pos()> or C<\g> or C</c> all over the place.

[Update: That turns out to be the C<:p> modifier.]

=head1 C<< <cut> >>

Well, I could say a lot more, but that's it for this time. I hope
you're excited by all this, in a positive sort of way. But if your jaw
lost all of its bounce when it hit the table, I expect Damian's
upcoming Exegesis 5 will do a better job of showing how this all fits
together into a pretty picture.
