=encoding utf8

=head1 NAME

Synopsis_05 - Rules

=head1 AUTHORS

Damian Conway <damian@conway.org> and
Allison Randal <al@shadowed.net>

=head1 VERSION

   Maintainer: Patrick Michaud <pmichaud@pobox.com>
   Date: 24 Jun 2002
   Last Modified: 16 Nov 2005
   Number: 5
   Version: 8

This document summarizes Apocalypse 5, which is about the new regex
syntax.  We now try to call them "rules" because they haven't been
regular expressions for a long time.  (The term "regex" is still
acceptable.)

=head1 New match state and capture variables

The underlying match state object is now available as the C<$/>
variable, which is implicitly lexically scoped.  All access to the
current (or most recent) match is through this variable, even when
it doesn't look like it.  The individual capture variables (such as C<$0>,
C<$1>, etc.) are just elements of C<$/>.

By the way, the numbered capture variables now start at C<$0>, C<$1>,
C<$2>, etc. See below.

=head1 Unchanged syntactic features

The following regex features use the same syntax as in Perl 5:

=over

=item *

Capturing: (...)

=item *

Repetition quantifiers: *, +, and ?

=item *

Alternatives:  |

=item *

Backslash escape:  \

=item *

Minimal matching suffix:   ??,  *?,  +?

=back

=head1 Modifiers

=over

=item *

The extended syntax (C</x>) is no longer required...it's the default.

=item *

There are no C</s> or C</m> modifiers (changes to the meta-characters
replace them - see below).

=item *

There is no C</e> evaluation modifier on substitutions; instead use:

     s/pattern/{ code() }/

=item *

Modifiers are now placed as adverbs at the I<start> of a match/substitution:

     m:g:i/\s* (\w*) \s* ,?/;

Every modifier must start with its own colon.  The delimiter must be
separated from the final modifier by a colon or whitespace if it would
be taken as an argument to the preceding modifier.

=item *

The single-character modifiers also have longer versions:

         :i        :ignorecase
         :g        :global

=item *

The C<:c> (or C<:continue>) modifier causes the pattern to continue
scanning from the string's current C<.pos>:

     m:c/ pattern /        # start at end of
                           # previous match on $_

Note that this does not automatically anchor the pattern to the starting
location.  (Use C<:p> for that.)  The pattern you supply to C<split>
has an implicit C<:c> modifier.

=item *

The C<:p> (or C<:pos>) modifier causes the pattern to try to match only at
the string's current C<.pos>:

     m:p/ pattern /        # match at end of
                           # previous match on $_

Since this is implicitly anchored to the position, it's suitable for
building parsers and lexers.  The pattern you supply to a Perl macro's
"is parsed" trait has an implicit C<:p> modifier.

Note that

     m:c/pattern/

is roughly equivalent to

     m:p/.*? pattern/

=item *

The new C<:once> modifier replaces the Perl 5 C<?...?> syntax:

     m:once/ pattern /    # only matches first time

=item *

[Note: We're still not sure if :w is ultimately going to work exactly 
as described below.  But this is how it works for now.]

The new C<:w> (C<:words>) modifier causes whitespace sequences to be
replaced by C<\s*> or C<\s+> subpattern as defined by the C<< <?ws> >> rule.

     m:w/ next cmd =   <condition>/

Same as:

     m/ <?ws> next <?ws> cmd <?ws> = <?ws> <condition>/

which is effectively the same as:

     m/ \s* next \s+ cmd \s* = \s* <condition>/

But in the case of

     m:w { (a|\*) (b|\+) }

or equivalently,

     m { (a|\*) <?ws> (b|\+) }

C<< <?ws> >> can't decide what to do until it sees the data.  It still does
the right thing.  If not, define your own C<< <?ws> >> and C<:w> will use that.

=item *

New modifiers specify Unicode level:

     m:bytes / .**{2} /       # match two bytes
     m:codes / .**{2} /       # match two codepoints
     m:graphs/ .**{2} /       # match two graphemes
     m:langs / .**{2} /       # match two language dependent chars

There are corresponding pragmas to default to these levels.

=item *

The new C<:perl5> modifier allows Perl 5 regex syntax to be used instead:

     m:perl5/(?mi)^[a-z]{1,2}(?=\s)/

(It does not go so far as to allow you to put your modifiers at
the end.)

=item *

Any integer modifier specifies a count. What kind of count is
determined by the character that follows.

=item *

If followed by an C<x>, it means repetition.  Use C<:x(4)> for the
general form.  So

     s:4x { (<?ident>) = (\N+) $$}{$0 => $1};

is the same as:

     s:x(4) { (<?ident>) = (\N+) $$}{$0 => $1};

which is almost the same as:

     $_.pos = 0;
     s:c{ (<?ident>) = (\N+) $$}{$0 => $1} for 1..4;

except that the string is unchanged unless all four matches are found.
However, ranges are allowed, so you can say C<:x(1..4)> to change anywhere
from one to four matches.

=item *

If the number is followed by an C<st>, C<nd>, C<rd>, or C<th>, it means
find the I<N>th occurrence.  Use C<:nth(3)> for the general form.  So

     s:3rd/(\d+)/@data[$0]/;

is the same as

     s:nth(3)/(\d+)/@data[$0]/;

which is the same as:

     m/(\d+)/ && m:c/(\d+)/ && s:c/(\d+)/@data[$0]/;

Lists and junctions are allowed: C<:nth(1|2|3|5|8|13|21|34|55|89)>.

So are closures: C<:nth{.is_fibonacci}>

=item *

With the new C<:ov> (C<:overlap>) modifier, the current rule will
match at all possible character positions (including overlapping)
and return all matches in a list context, or a disjunction of matches
in a scalar context.  The first match at any position is returned.

     $str = "abracadabra";

     if $str ~~ m:overlap/ a (.*) a / {
         @substrings = $/.matches();    # bracadabr cadabr dabr br
     }

=item *

With the new C<:ex> (C<:exhaustive>) modifier, the current rule will match
every possible way (including overlapping) and return all matches in a list
context, or a disjunction of matches in a scalar context.

     $str = "abracadabra";

     if $str ~~ m:exhaustive/ a (.*) a / {
         @substrings = $/.matches();    # br brac bracad bracadabr
                                        # c cad cadabr d dabr br
     }


=item *

The new C<:rw> modifier causes this rule to "claim" the current
string for modification rather than assuming copy-on-write semantics.
All the bindings in C<$/> become lvalues into the string, such
that if you modify, say, C<$1>, the original string is modified in
that location, and the positions of all the other fields modified
accordingly (whatever that means).  In the absence of this modifier
(especially if it isn't implemented yet, or is never implemented),
all pieces of C<$/> are considered copy-on-write, if not read-only.

=item *

The new C<:keepall> modifier causes this rule and all invoked subrules
to remember everything, even if the rules themselves don't ask for
their subrules to be remembered.  This is for forcing a grammar that
throws away whitespace and comments to keep them instead.

=item *

The C<:i>, C<:w>, C<:perl5>, and Unicode-level modifiers can be
placed inside the rule (and are lexically scoped):

     m/:w alignment = [:i left|right|cent[er|re]] /

=item *

User-defined modifiers will be possible:

         m:fuzzy/pattern/;

=item *

User-defined modifiers can also take arguments:

         m:fuzzy('bare')/pattern/;

=item *

To use parens or brackets for your delimiters you have to separate:

         m:fuzzy (pattern);
         m:fuzzy:(pattern);

or you'll end up with:

         m:fuzzy(fuzzyargs); pattern ;

=back

=head1 Changed metacharacters

=over

=item *

A dot C<.> now matches I<any> character including newline. (The C</s>
modifier is gone.)

=item *

C<^> and C<$> now always match the start/end of a string, like the
old C<\A> and C<\z>. (The C</m> modifier is gone.)

=item *

A C<$> no longer matches an optional preceding C<\n> so it's necessary
to say C<\n?$> if that's what you mean.

=item *

C<\n> now matches a logical (platform independent) newline not just C<\x0a>.

=item *

The C<\A>, C<\Z>, and C<\z> metacharacters are gone.

=back

=head1 New metacharacters

=over

=item *

Because C</x> is default:

=over

=item *

C<#> now always introduces a comment.

=item *

Whitespace is now always metasyntactic, i.e. used only for layout
and not matched literally (but see the C<:w> modifier described above).

=back

=item *

C<^^> and C<$$> match line beginnings and endings. (The C</m>
modifier is gone.)  They are both zero-width assertions.  C<$$>
matches before any C<\n> (logical newline), and also at the end of
the string if the final character was I<not> a C<\n>.  C<^^> always
matches the beginning of the string and after any C<\n> that is not
the final character in the string.

=item *

C<.> matches an "anything", while C<\N> matches an "anything except
newline". (The C</s> modifier is gone.)  In particular, C<\N> matches
neither carriage return nor line feed.

=item *

The new C<&> metacharacter separates conjunctive terms.  The patterns on
either side must match with the same beginning and end point.  The
operator is list associative like C<|>, has higher precedence than C<|>,
and backtracking makes the right argument vary faster than the left.   

=back

=head1 Bracket rationalization

=over

=item *

C<(...)> still delimits a capturing group. However the ordering of these
groups is hierarchical, rather than linear. See L<Nested subpattern captures>.

=item *

C<[...]> is no longer a character class.
It now delimits a non-capturing group.

=item *

C<{...}> is no longer a repetition quantifier.
It now delimits an embedded closure.

=item *

You can call Perl code as part of a rule match by using a closure.
Embedded code does not usually affect the match--it is only used
for side-effects:

     / (\S+) { print "string not blank\n"; $text = $0; }
        \s+  { print "but does contain whitespace\n" }
     /

=item *

It can affect the match if it calls C<fail>:

     / (\d+) { $0 < 256 or fail } /

Closures are guaranteed to be called at the canonical time even if
the optimizer could prove that something after them can't match.
(Anything before is fair game, however.)

=item *

The repetition specifier is now C<**{...}> for maximal matching,
with a corresponding C<**{...}?> for minimal matching.  Space is
allowed on either side of the asterisks.  The curlies are taken to
be a closure returning a number or a range.

     / value was (\d ** {1..6}?) with ([\w]**{$m..$n}) /

It is illegal to return a list, so this easy mistake fails:

     / [foo]**{1,3} /

(At least, it fails in the absence of "C<use rx :listquantifier>",
which is likely to be unimplemented in Perl 6.0.0 anyway).

The optimizer will likely optimize away things like C<**{1...}>
so that the closure is never actually run in that case.  But it's
a closure that must be run in the general case, so you can use
it to generate a range on the fly based on the earlier matching.
(Of course, bear in mind the closure is run I<before> attempting to
match whatever it quantifies.)

=item *

C<< <...> >> are now extensible metasyntax delimiters or "assertions"
(i.e. they replace Perl 5's crufty C<(?...)> syntax).

=back

=head1 Variable (non-)interpolation

=over

=item *

In Perl 6 rules, variables don't interpolate.

=item *

Instead they're passed "raw" to the rule engine, which can then decide
how to handle them (more on that below).

=item *

The default way in which the engine handles a scalar is to match it
as a C<< <'...'> >> literal (i.e.it does not treat the interpolated string
as a subpattern).  In other words, a Perl 6:

     / $var /

is like a Perl 5:

     / \Q$var\E /

(To get rule interpolation use an assertion - see below)

=item *

An interpolated array:

     / @cmds /

is matched as if it were an alternation of its elements:

     / [ @cmds[0] | @cmds[1] | @cmds[2] | ... ] /


As with a scalar variable, each one is matched as a literal.

=item *

An interpolated hash matches the longest possible key of the hash
as a literal, or fails if no key matches.  (A C<""> key
will match anywhere, provided no longer key matches.)

=over 4

=item *

If the corresponding value of the hash element is a closure, it
is executed.

=item *

If it is a string or rule object, it is executed as a subrule.

=item *

If it has the value 1, nothing special happens beyond the match.

=item *

Any other value causes the match to fail.

=back

=back

=head1 Extensible metasyntax (C<< <...> >>)

=over

=item *

The first character after C<< < >> determines the behaviour of the assertion.

=item *

A leading alphabetic character means it's a capturing grammatical 
assertion (i.e. a subrule or a named character class - see below):

     / <sign>? <mantissa> <exponent>? /

=item *

The special named assertions include:

     / <before pattern> /    # was /(?=pattern)/
     / <after pattern> /     # was /(?<pattern)/

     / <ws> /                # match whitespace by :w rules

     / <sp> /                # match a space char

The C<after> assertion implements lookbehind by reversing the syntax
tree and looking for things in the opposite order going to the left.
It is illegal to do lookbehind on a pattern that cannot be reversed.

=item *

A leading C<?> causes the assertion not to capture what it matches (see
L<Subrule captures>. For example:

     / <ident>  <ws>  /      # $/<ident> and $/<ws> both captured
     / <?ident> <ws>  /      # only $/<ws> captured
     / <?ident> <?ws> /      # nothing captured

=item *

A leading C<$> indicates an indirect rule.  The variable must contain
either a hard reference to a rule, or a string containing the rule.

=item *

A leading C<::> indicates a symbolic indirect rule:

     / <::($somename)>

The variable must contain the name of a rule.

=item *

A leading C<@> matches like a bare array except that each element
is treated as a rule (string or hard ref) rather than as a literal.

=item *

A leading C<%> matches like a bare hash except that each key
is treated as a rule (string or hard ref) rather than as a literal.

=item *

A leading C<{> indicates code that produces a rule to be interpolated
into the pattern at that point:

     / (<?ident>)  <{ %cache{$0} //= get_body($0) }> /

The closure is guaranteed to be run at the canonical time.

=item *

A leading C<&> interpolates the return value of a subroutine call as
a rule.  Hence

     <&foo()>

is short for

     <{ foo() }>

=item *

In any case of rule interpolation, if the value already happens to be
a rule object, it is not recompiled.  If it is a string, the compiled
form is cached with the string so that it is not recompiled next
time you use it unless the string changes.  (Any external lexical
variable names must be rebound each time though.)  Rules may not be
interpolated with unbalanced bracketing.  An interpolated subrule
keeps its own inner C<$/>, so its parentheses never count toward the
outer rules groupings.  (In other words, parenthesis numbering is always
lexically scoped.)

=item *

A leading C<(> indicates a code assertion:

     / (\d**{1..3}) <( $0 < 256 )> /

Similar to:

     / (\d**{1..3}) { $0 < 256 or fail } /

Unlike closures, code assertions are not guaranteed to be run at the
canonical time if the optimizer can prove something later can't match.
So you can sneak in a call to a non-canonical closure that way:

     /^foo .* <( do { say "Got here!" } or 1 )> .* bar$/

The C<do> block is unlikely to run unless the string ends with "C<bar>".

=item *

A leading C<[> or C<+> indicates an enumerated character class.  Ranges
in enumerated character classes are indicated with C<..>.

     / <[a..z_]>* /
     / <+[a..z_]>* /

=item *

A leading C<-> indicates a complemented character class:

     / <-[a..z_]> <-alpha> /

=item *

Character classes can be combined (additively or subtractively) within
a single set of angle brackets. For example:

     / <[a..z]-[aeiou]+xdigit> /      # consonant or hex digit

If such a combination starts with a named character class, a leading
C<+> is required:

     / <+alpha-[Jj]> /               # J-less alpha

=item *

A leading C<'> indicates a literal match (including whitespace):

     / <'match this exactly (whitespace matters)'> /

=item *

A leading C<"> indicates a literal match after interpolation:

     / <"match $THIS exactly (whitespace still matters)"> /

=item *

The special assertion C<< <.> >> matches any logical grapheme
(including a Unicode combining character sequences):

     / seekto = <.> /  # Maybe a combined char

Same as:

     / seekto = [:graphs .] /

=item *

A leading C<!> indicates a negated meaning (always a zero-width assertion):

     / <!before _ > /    # We aren't before an _

=back

=head1 Backslash reform

=over

=item *

The C<\p> and C<\P> properties become intrinsic grammar rules
(C<< <prop ...> >> and C<< <!prop ...> >>).

=item *

The C<\L...\E>, C<\U...\E>, and C<\Q...\E> sequences are gone.  In the
rare cases that need them you can use C<< <{ lc $rule }> >> etc.

=item *

The C<\G> sequence is gone.  Use C<:p> instead.  (Note, however, that
it makes no sense to use C<:p> within a pattern, since every internal
pattern is implicitly anchored to the current position.  You'll have
to explicitly compare C<< <( .pos == $oldpos )> >> in that case.)

=item *

Backreferences (e.g. C<\1>, C<\2>, etc.) are gone; C<$0>, C<$1>, etc. can be
used instead, because variables are no longer interpolated.

=item *

New backslash sequences, C<\h> and C<\v>, match horizontal and vertical
whitespace respectively, including Unicode.

=item *

C<\s> now matches any Unicode whitespace character.

=item *

The new backslash sequence C<\N> matches anything except a logical
newline; it is the negation of C<\n>.

=item *

A series of other new capital backslash sequences are also the negation
of their lower-case counterparts:

=over

=item *

C<\H> matches anything but horizontal whitespace.

=item *

C<\V> matches anything but vertical whitespace.

=item *

C<\T> matches anything but a tab.

=item *

C<\R> matches anything but a return.

=item *

C<\F> matches anything but a formfeed.

=item *

C<\E> matches anything but an escape.

=item *

C<\X...> matches anything but the specified character (specified in
hexadecimal).

=back

=back

=head1 Regexes are rules

=over

=item *

The Perl 5  C<qr/pattern/> regex constructor is gone.

=item *

The Perl 6 equivalents are:

     rule { pattern }    # always takes {...} as delimiters
       rx / pattern /    # can take (almost any) chars as delimiters

You may not use whitespace or alphanumerics for delimiters.  Space is
optional unless needed to distinguish from modifier arguments or
function parens.  So you may use parens as your C<rx> delimiters,
but only if you interpose a colon or whitespace:

     rx:( pattern )      # okay
     rx ( pattern )      # okay
     rx( 1,2,3 )         # tries to call rx function

=item *

If either form needs modifiers, they go before the opening delimiter:

     $rule = rule :g:w:i { my name is (.*) };
     $rule = rx:g:w:i / my name is (.*) /;

Space or colon is necessary after the final modifer if you use any
bracketing character for the delimiter.  (Otherwise it would be taken as
an argument to the modifier.)

=item *

You may not use colons for the delimiter.  Space is allowed between
modifiers:

     $rule = rx :g :w :i / my name is (.*) /;

=item *

The name of the constructor was changed from C<qr> because it's no
longer an interpolating quote-like operator.  C<rx> stands for "rule
expression", or occasionally "regex".  C<:-)>

=item *

As the syntax indicates, it is now more closely analogous to a C<sub {...}>
constructor.  In fact, that analogy will run I<very> deep in Perl 6.

=item *

Just as a raw C<{...}> is now always a closure (which may still
execute immediately in certain contexts and be passed as a reference
in others), so too a raw C</.../> is now always a rule (which may still
match immediately in certain contexts and be passed as a reference
in others).

=item *

Specifically, a C</.../> matches immediately in a value context (void,
Boolean, string, or numeric), or when it is an explicit argument of
a C<~~>.  Otherwise it's a rule constructor.  So this:

     $var = /pattern/;

no longer does the match and sets C<$var> to the result.
Instead it assigns a rule reference to C<$var>.

=item *

The two cases can always be distinguished using C<m{...}> or C<rx{...}>:

     $var = m{pattern};    # Match rule immediately, assign result
     $var = rx{pattern};   # Assign rule expression itself

=item *

Note that this means that former magically lazy usages like:

     @list = split /pattern/, $str;

are now just consequences of the normal semantics.

=item *

It's now also possible to set up a user-defined subroutine that acts
like C<grep>:

     sub my_grep($selector, *@list) {
         given $selector {
             when Rule  { ... }
             when Code  { ... }
             when Hash  { ... }
             # etc.
         }
     }

Using C<{...}> or C</.../> in the scalar context of the first argument
causes it to produce a C<Code> or C<Rule> reference, which the switch
statement then selects upon.

=back

=head1 Backtracking control

=over

=item *

Backtracking over a single colon causes the rule engine not to retry
the preceding atom:

     m:w/ \( <expr> [ , <expr> ]* : \) /

(i.e. there's no point trying fewer C<< <expr> >> matches, if there's
no closing parenthesis on the horizon)

=item *

Backtracking over a double colon causes the surrounding group of
alternations to immediately fail:

     m:w/ [ if :: <expr> <block>
          | for :: <list> <block>
          | loop :: <loop_controls>? <block>
          ]
     /

(i.e. there's no point trying to match a different keyword if one
was already found but failed).

=item *

Backtracking over a triple colon causes the current rule to fail
outright (no matter where in the rule it occurs):

     rule ident {
           ( [<alpha>|_] \w* ) ::: { fail if %reserved{$0} }
         | " [<alpha>|_] \w* "
     }

     m:w/ get <ident>? /

(i.e. using an unquoted reserved word as an identifier is not permitted)

=item *

Backtracking over a C<< <commit> >> assertion causes the entire match
to fail outright, no matter how many subrules down it happens:

     rule subname {
         ([<alpha>|_] \w*) <commit> { fail if %reserved{$0} }
     }
     m:w/ sub <subname>? <block> /

(i.e. using a reserved word as a subroutine name is instantly fatal
to the "surrounding" match as well)

=item *

A C<< <cut> >> assertion always matches successfully, and has the
side effect of deleting the parts of the string already matched.

=item *

Attempting to backtrack past a C<< <cut> >> causes the complete match
to fail (like backtracking past a C<< <commit> >>. This is because there's
now no preceding text to backtrack into.

=item *

This is useful for throwing away successfully processed input when
matching from an input stream or an iterator of arbitrary length.

=back

=head1 Named Regexes

=over

=item *

The analogy between C<sub> and C<rule> extends much further.

=item *

Just as you can have anonymous subs and named subs...

=item *

...so too you can have anonymous rules and I<named> rules:

     rule ident { [<alpha>|_] \w* }

     # and later...

     @ids = grep /<ident>/, @strings;

=item *

As the above example indicates, it's possible to refer to named rules,
such as:

     rule serial_number { <[A..Z]> \d**{8} }
     rule type { alpha | beta | production | deprecated | legacy }

in other rules as named assertions:

     rule identification { [soft|hard]ware <type> <serial_number> }

=back

=head1 Nothing is illegal

=over

=item *

The null pattern is now illegal.

=item *

To match whatever the prior successful rule matched, use:

     /<prior>/

=item *

To match the zero-width string, use:

     /<null>/

For example:

     split /<?null>/, $string

splits between characters.

=item *

To match a null alternative, use:

     /a|b|c|<?null>/

This makes it easier to catch errors like this:

     m:w/ [
          | if :: <expr> <block>
          | for :: <list> <block>
          | loop :: <loop_controls>? <block>
          ]
     /

=item *

However, it's okay for a non-null syntactic construct to have a degenerate
case matching the null string:

     $something = "";
     /a|b|c|$something/;

=back

=head1 Return values from matches

=head2 Match objects

=over

=item *

A match always returns a "match object", which is also available as
(lexical) C<$/> (except within a closure lexically embedded in a rule,
where C<$/> always refers to the current match, not any submatch done
within the closure).

=item *

The match object evaluates differently in different contexts:

=over

=item *

In boolean context it evaluates as true or false (i.e. did the match
succeed?):

     if /pattern/ {...}
     # or:
     /pattern/; if $/ {...}

=item *

In string context it evaluates to the entire matched string:

     print %hash{ "{$text ~~ /<?ident>/}" };
     # or equivalently:
     $text ~~ /<?ident>/  &&  print %hash{~$/};

But generally you should say C<~$/> if you mean C<~$/>.

=item *

In numeric context it evaluates to the numeric value of the entire matched
string:

     $sum += /\d+/;
     # or equivalently:
     /\d+/; $sum = $sum + $/;

=item *

When used as an array, C<$/> pretends to be an array containing
C<$0>, C<$1>, etc.  Hence

     ($key, $val) = m:w/ (\S+) => (\S+)/;

can also be written:

     $result = m:w/ (\S+) => (\S+)/;
     ($key, $val) = @$result;

To get a single capture into a string, use a subscript:

     $mystring = "{ m:w/ (\S+) => (\S+)/[0] }";

To get all the captures into a string, use a "zen" slice:

     $mystring = "{ m:w/ (\S+) => (\S+)/[] }";

Note that, as a scalar variable, C<$/> doesn't automatically flatten
in list context.  Use C<@$/> or C<$/[]> to flatten as an array.

=item *

When used as a hash, C<$/> pretends to be a hash of all the named
captures.  The keys do not include any sigils, so if you capture to
variable C<< @<foo> >> its real name is C<$/{'foo'}> or C<< $/<foo> >>.
However, you may still refer to it as C<< @<foo> >> anywhere C<$/>
is visible.  (But it is erroneous to use the same name for two different
capture datatypes.)

Note that, as a scalar variable, C<$/> doesn't automatically flatten
in list context.  Use C<%$/> or C<$/{}> to flatten as a hash, or bind
it to a variable of the appropriate type.

=item *

The numbered captures may be treated as named, so C<< $<0 1 2> >>
is equivalent to C<$/[0,1,2]>.  This allows you to write slices of
intermixed named and numbered captures.

=item *

In ordinary code, variables C<$0>, C<$1>, etc. are just aliases into
C<$/[0]>, C<$/[1]>, etc.  Hence they will all be undefined if the
last match failed (unless they were explicitly bound in a closure without
using the C<let> keyword).

=back

=item *

C<Match> objects have methods that provide additional information about
the match. For example:

     if m/ def <ident> <codeblock> / {
         say "Found sub def from index $/.from() to index $/.to()";
     }

=item *

All match attempts--successful or not--against any rule, subrule, or
subpattern (see below) return an object of class C<Match>. That is:

     $match_obj = $str ~~ /pattern/;
     say "Matched" if $match_obj;

=item *

This returned object is also automatically assigned to the lexical
C<$/> variable, unless the match statement is inside another rule. That is:

     $str ~~ /pattern/;
     say "Matched" if $/;

=item *

Inside a rule, the C<$/> variable holds the current rule's
incomplete C<Match> object (which can be modified via the internal C<$/>.
For example:

     $str ~~ / foo                # Match 'foo'
               { $/ = 'bar' }     # But pretend we matched 'bar'
             /;

=back

=head2 Subpattern captures

=over

=item *

Any part of a rule that is enclosed in capturing parentheses is called a
I<subpattern>. For example:

        #               subpattern
        #  _________________/\____________________
        # |                                       |
        # |       subpattern  subpattern          |
        # |          __/\__    __/\__             |
        # |         |      |  |      |            |
     m:w/ (I am the (walrus), ( khoo )**{2} kachoo) /;


=item *

Each subpattern in a rule produces a C<Match> object if it is
successfully matched.

=item *

Each subpattern's C<Match> object is pushed onto the array inside
the outer C<Match> object belonging to the surrounding scope (known as
its I<parent C<Match> object>). The surrounding scope may be either the
innermost surrounding subpattern (if the subpattern is nested) or else
the entire rule itself.

=item *

Like all captures, these assignments to the array are hypothetical, and
are undone if the subpattern is backtracked.

=item *

For example, if the following pattern matched successfully:

        #                subpat-A
        #  _________________/\____________________
        # |                                       |
        # |         subpat-B  subpat-C            |
        # |          __/\__    __/\__             |
        # |         |      |  |      |            |
     m:w/ (I am the (walrus), ( khoo )**{2} kachoo) /;

then the C<Match> objects representing the matches made by I<subpat-B>
and I<subpat-C> would be successively pushed onto the array inside I<subpat-
A>'s C<Match> object. Then I<subpat-A>'s C<Match> object would itself be
pushed onto the array inside the C<Match> object for the entire rule
(i.e. onto C<$/>'s array).

=item *

As a result of these semantics, capturing parentheses in Perl 6 are
hierarchical, not linear (see L<Nested subpattern captures>).

=back

=head2 Accessing captured subpatterns

=over

=item *

The array elements of a C<Match> object are referred to using either the
standard array access notation (e.g. C<$/[0]>, C<$/[1]>, C<$/[2]>, etc.)
or else via the corresponding lexically scoped numeric aliases (i.e.
C<$0>, C<$1>, C<$2>, etc.) So:

     say "$/[1] was found between $/[0] and $/[2]";

is the same as:

     say "$1 was found between $0 and $2";

=item *

Note that, in Perl 6, the numeric capture variables start from $0, not
$1, with the numbers corresponding to the element's index inside C<$/>.

=item *

The array elements of the rule's C<Match> object (i.e. C<$/>)
store individual C<Match> objects representing the substrings that where
matched and captured by the first, second, third, etc. I<outermost>
(i.e. unnested) subpatterns. So these elements can be treated like fully
fledged match results. For example:

     if m/ (\d\d\d\d)-(\d\d)-(\d\d) (BCE?|AD|CE)?/ {
           ($yr, $mon, $day) = $/[0..2]
           $era = "$3" if $3;                    # stringify/boolify
           @datepos = ( $0.from() .. $2.to() );  # Call Match methods
     }


=back

=head2 Nested subpattern captures

=over

=item *

Substrings matched by I<nested> subpatterns (i.e. nested capturing
parens) are assigned to the array inside the subpattern's parent C<Match>
surrounding subpattern, not to the array of C<$/>.

=item *

This behaviour is quite different to Perl 5 semantics:

      # Perl 5...
      #
      # $1---------------------  $4---------  $5------------------
      # |   $2---------------  | |          | | $6----  $7------  |
      # |   |         $3--   | | |          | | |     | |       | |
      # |   |         |   |  | | |          | | |     | |       | |
     m/ ( A (guy|gal|g(\S+)  ) ) (sees|calls) ( (the|a) (gal|guy) ) /x;

=item *

In Perl 6, nested parens produce properly nested captures:

      # Perl 6...
      #
      # $0---------------------  $1---------  $2------------------
      # |   $0[0]------------  | |          | | $2[0]-  $2[1]---  |
      # |   |       $0[0][0] | | |          | | |     | |       | |
      # |   |         |   |  | | |          | | |     | |       | |
     m/ ( A (guy|gal|g(\S+)  ) ) (sees|calls) ( (the|a) (gal|guy) ) /;


=back

=head2 Quantified subpattern captures

=over

=item *

If a subpattern is directly quantified (using I<any> quantifier), it no
longer produces a single C<Match> object. Instead, it produces a list
of C<Match> objects corresponding to the sequence of individual matches
made by the repeated subpattern.

=item *

Because a quantified subpattern returns a list of C<Match> objects, the
corresponding array element for the quantified capture will store a
reference to a (nested) array, rather than a single C<Match> object.
For example:

     if m/ (\w+) \: (\w+ \s+)* / {
         say "Key:    $0";         # Unquantified --> single Match
         say "Values: { @{$1} }";  # Quantified   --> array of Match
     }


=back

=head2 Indirectly quantified subpattern captures

=over

=item *

A subpattern may sometimes be nested inside a quantified non-capturing
structure:

      #       non-capturing       quantifier
      #  __________/\____________  __/\__
      # |                        ||      |
      # |   $0         $1        ||      |
      # |  _^_      ___^___      ||      |
      # | |   |    |       |     ||      |
     m/ [ (\w+) \: (\w+ \h+)* \n ]**{2...} /

Non-capturing brackets I<don't> create a separate nested lexical scope,
so the two subpatterns inside them are actually still in the rule's
top-level scope. Hence their top-level designations: C<$0> and C<$1>.

=item *

However, because the two subpatterns are inside a quantified
structure, C<$0> and C<$1> will each contain a reference to an array.
The elements of that array will be the submatches returned by the
corresponding subpattern on each iteration of the non-capturing
parentheses. For example:

     my $text = "foo:food fool\nbar:bard barb";

               #   $0--     $1------
               #   |   |    |       |
     $text ~~ m/ [ (\w+) \: (\w+ \h+)* \n ]**{2...} /;

     # Because they're in a quantified non-capturing block...
     # $0 contains the equivalent of:
     #
     #       [ Match.new(str=>'foo'), Match.new(str=>'bar') ]
     #
     # and $1 contains the equivalent of:
     #
     #       [ Match.new(str=>'food '),
     #         Match.new(str=>'fool' ),
     #         Match.new(str=>'bard '),
     #         Match.new(str=>'barb' ),
     #       ]


=item *

In contrast, if the outer quantified structure is a I<capturing>
structure (i.e. a subpattern) then it I<will> introduce a nested
lexical scope. That outer quantified structure will then
return an array of C<Match> objects representing the captures
of the inner parens for I<every> iteration (as described above). That is:

     my $text = "foo:food fool\nbar:bard barb";

               # $0-----------------------
               # |                        |
               # | $0[0]    $0[1]---      |
               # | |   |    |       |     |
     $text ~~ m/ ( (\w+) \: (\w+ \h+)* \n )**{2...} /;

     # Because it's in a quantified capturing block,
     # $0 contains the equivalent of:
     #
     #       [ Match.new( str=>"foo:food fool\n",
     #                    arr=>[ Match.new(str=>'foo'),
     #                           [
     #                               Match.new(str=>'food '),
     #                               Match.new(str=>'fool'),
     #                           ]
     #                         ],
     #                  ),
     #         Match.new( str=>'bar:bard barb',
     #                    arr=>[ Match.new(str=>'bar'),
     #                           [
     #                               Match.new(str=>'bard '),
     #                               Match.new(str=>'barb'),
     #                           ]
     #                         ],
     #                  ),
     #       ]
     #
     # and there is no $1

=item *

In other words, quantified non-capturing parens collect their components
into handy flattened lists, whereas quantified capturing parens collect
their components in a handy hierarchical structure.

=back

=head2 Subpattern numbering

=over

=item *

The index of a given subpattern can always be statically determined, but
is not necessarily unique nor always monotonic. The numbering of subpatterns
restarts in each lexical scope (either a rule, a subpattern, or the
branch of an alternation).

=item *

In particular, the index of capturing parentheses restarts after each
C<|>. Hence:

                  # $0      $1    $2   $3    $4           $5
     $tune_up = rx/ (don't) (ray) (me) (for) (solar tea), (d'oh!)
                  # $0      $1      $2    $3        $4
                  | (every) (green) (BEM) (devours) (faces)
                  /;

This means that if the second alternation matches, the C<@$/> array will
contain C<('every', 'green', 'BEM', 'devours', 'faces')>, rather than
C<(undef, undef, undef, undef, undef, undef, 'every', 'green', 'BEM',
'devours', 'faces')> (as the same regex would in Perl 5).

=item *

Note that it is still possible to mimic the monotonic Perl 5 capture
indexing semantics.  See L<Numbered scalar aliasing> below for details.


=back

=head2 Subrule captures

=over

=item *

Any call to a named C<< <rule> >> within a pattern is known as a I<subrule>.

=item *

Any bracketed construct that is aliased (see L<Aliasing> below) to a
named variable is also a subrule.

=item *

For example, this rule contains three subrules:

      # subrule       subrule      subrule
      #  __^__    _______^______    __^__
      # |     |  |              |  |     |
     m/ <ident>  $<spaces>:=(\s*)  <digit>+ /

=item *

Just like subpatterns, each successfully matched subrule within a rule
produces a C<Match> object. But, unlike subpatterns, that C<Match>
object is not assigned to the array inside its parent C<Match> object.
Instead, it is assigned to an entry of the hash inside its parent C<Match>
object. For example:

      #  .... $/ .....................................
      # :                                             :
      # :              .... $/[0] ..................  :
      # :             :                             : :
      # : $/<ident>   :        $/[0]<ident>         : :
      # :   __^__     :           __^__             : :
      # :  |     |    :          |     |            : :
     m:w/  <ident> \: ( known as <ident> previously ) /


=back

=head2 Accessing captured subrules

=over

=item *

The hash entries of a C<Match> object can be referred to using any of the
standard hash access notations (C<$/{'foo'}>, C<< $/<bar> >>, C<$/«baz»>,
etc.), or else via corresponding lexically scoped aliases (C<< $<foo> >>,
C<$«bar»>, C<< $<baz> >>, etc.)  So the previous example also implies:

      #    $<ident>             $0<ident>
      #     __^__                 __^__
      #    |     |               |     |
     m:w/  <ident> \: ( known as <ident> previously ) /

=item *

Note that it makes no difference whether a subrule is angle-bracketted (C<<
<ident> >>) or aliased (C<< $<ident> := (<alpha>\w*) >>. The name's the thing.


=back

=head2 Repeated captures of the same subrule

=over

=item *

If a subrule appears two (or more) times in the same lexical scope
(i.e. twice within the same subpattern and alternation), or if the
subrule is quantified anywhere within the entire rule, then its
corresponding hash entry is always assigned a reference to an array of
C<Match> objects, rather than a single C<Match> object.

=item *

Successive matches of the same subrule (whether from separate calls, or
from a single quantified repetition) append their individual C<Match>
objects to this array. For example:

     if m:w/ mv <file> <file> / {
         $from = $<file>[0];
         $to   = $<file>[1];
     }

Likewise, with a quantified subrule:

     if m:w/ mv <file>**{2} / {
         $from = $<file>[0];
         $to   = $<file>[1];
     }

Likewise, with a mixture of both:

     if m:w/ mv <file>+ <file> / {
         $to   = pop @{$<file>};
         @from = @{$<file>};
     }

=item *

However, if a subrule is explicitly renamed (or aliased -- see L<Aliasing>),
then only the "final" name counts when deciding whether it is or isn't
repeated. For example:

     if m:w/ mv <file> $<dir>:=<file> / {
         $from = $<file>;  # Only one subrule named <file>, so scalar
         $to   = $<dir>;   # The Capture Formerly Known As <file>
     }


Likewise, neither of the following constructions causes C<< <file> >> to
produce an array of C<Match> objects, since none of them has two or more
C<< <file> >> subrules in the same lexical scope:

     if m:w/ (keep) <file> | (toss) <file> / {
         # Each <file> is in a separate alternation, therefore <file>
         # is not repeated in any one scope, hence $<file> is
         # not an array ref...
         $action = $0;
         $target = $<file>;
     }

     if m:w/ <file> \: (<file>|none) / {
         # Second <file> nested in subpattern which confers a
         # different scope...
         $actual  = $/<file>;
         $virtual = $/[0]<file> if $/[0]<file>;
     }

=item *

On the other hand, unaliased square brackets don't confer a separate
scope (because they don't have an associated C<Match> object). So:

     if m:w/ <file> \: [<file>|none] / { # Two <file>s in same scope
         $actual  = $/<file>[0];
         $virtual = $/<file>[1] if $/<file>[1];
     }


=back

=head2 Aliasing

Aliases can be named or numbered. They can be scalar-, array-, or hash-like.
And they can be applied to either capturing or non-capturing constructs. The
following sections highlight special features of the semantics of some
of those combinations.


=head3 Named scalar aliasing to subpatterns

=over

=item *

If a named scalar alias is applied to a set of I<capturing> parens:

        #          ______/capturing parens\_____
        #         |                             |
        #         |                             |
     m:w/ $<key>:=( (<[A..E]>) (\d**{3..6}) (X?) ) /;

then the outer capturing parens no longer capture into the array of
C<$/> (like unaliased parens would). Instead the aliased parens capture
into the hash of C<$/>; specifically into the hash element
whose key is alias name.

=item *

So, in the above example, a successful match sets
C<< $<key> >> (i.e. C<< $/<key> >>), but I<not> C<$0> (i.e. C<< $/[0] >>).

=item *

More specifically:

=over

=item *

C<< $/<key> >> will contain the C<Match> object that would previously have
been placed in C<< $/[0] >>.

=item *

C<< $/<key>[0] >> will contain the A-E letter,

=item *

C<< $/<key>[1] >> will contain the digits,

=item *

C<< $/<key>[2] >> will contain the optional X.

=back

=item *

Another way to think about this behaviour is that aliased parens create
a kind of lexically scoped named subrule; that the contents of the
brackets are treated as if they were part of a separate subrule whose
name is the alias.


=back

=head3 Named scalar aliases applied to non-capturing brackets

=over

=item *

If an named scalar alias is applied to a set of I<non-capturing> brackets:

        #          ___/non-capturing brackets\__
        #         |                             |
        #         |                             |
     m:w/ $<key>:=[ (<[A..E]>) (\d**{3..6}) (X?) ] /;

then the corresponding C<< $/<key> >> object contains only the string
matched by the non-capturing brackets.

=item *

In particular, the array of the C<< $/<key> >> entry is empty. That's
because square brackets do not create a nested lexical scope, so the
subpatterns are unnested and hence correspond to $0, $1, and $2, and
I<not> to C<< $/<key>[0] >>, C<< $/<key>[1] >>, and C<< $/<key>[2] >>.

=item *

In other words:

=over

=item *

C<< $/<key> >> will contain the complete substring matched by the square
brackets (in a C<Match> object, as described above),

=item *

C<< $0 >> will contain the A-E letter,

=item *

C<< $1 >> will contain the digits,

=item *

C<< $2 >> will contain the optional X.

=back


=back

=head3 Named scalar aliasing to subrules

=over

=item *

If a subrule is aliased, it assigns its C<Match> object to the hash
entry whose key is the name of the alias. And it I<no longer> assigns
anything to the hash entry whose key is the subrule name. That is:

     if m:/ ID\: $<id>:=<ident> / {
         say "Identified as $/<id>";    # $/<ident> is undefined
     }

=item *

Hence aliasing a subrule I<changes> the destination of the subrule's C<Match>
object. This is particularly useful for differentiating two or more calls to
the same subrule in the same scope. For example:

     if m:w/ mv <file>+ $<dir>:=<file> / {
         @from = @{$<file>};
         $to   = $<dir>;
     }

=back

=head3 Numbered scalar aliasing

=over

=item *

If a numbered alias is used instead of a named alias:

     m/ $1:=(<-[:]>*) \:  $0:=<ident> /

the behaviour is exactly the same as for a named alias (i.e the various
cases described above), except that the resulting C<Match> object is
assigned to the corresponding element of the appropriate array, rather
than to an element of the hash.

=item *

If any numbered alias is used, the numbering of subsequent unaliased
subpatterns in the same scope automatically increments from that
alias number (much like enum values increment from the last explicit
value). That is:

      #  ---$1---    -$2-    ---$6---    -$7-
      # |        |  |    |  |        |  |    |
     m/ $1:=(food)  (bard)  $6:=(bazd)  (quxd) /;

=item *

This "follow-on" behaviour is particularly useful for reinstituting
Perl5 semantics for consecutive subpattern numbering in alternations:

     $tune_up = rx/ (don't) (ray) (me) (for) (solar tea), (d'oh!)
                  | $6:=(every) (green) (BEM) (devours) (faces)
                  #             $7      $8    $9        $10
                  /;

=item *

It also provides an easy way in Perl 6 to reinstitute the unnested
numbering semantics of nested Perl 5 subpatterns:

      # Perl 5...
      #               $1
      #  _____________/\______________
      # |    $2          $3       $4  |
      # |  __/\___   ____/\____   /\  |
      # | |       | |          | |  | |
     m/ ( (<[A..E]>) (\d**{3..6}) (X?) ) /;


      # Perl 6...
      #               $0
      #  _____________/\______________
      # |  $0[0]       $0[1]    $0[2] |
      # |  __/\___   ____/\____   /\  |
      # | |       | |          | |  | |
     m/ ( (<[A..E]>) (\d**{3..6}) (X?) ) /;


      # Perl 6 simulating Perl 5...
      #                 $1
      #  _______________/\________________
      # |        $2          $3       $4  |
      # |      __/\___   ____/\____   /\  |
      # |     |       | |          | |  | |
     m/ $1:=[ (<[A..E]>) (\d**{3..6}) (X?) ] /;

The non-capturing brackets don't introduce a scope, so the subpatterns within
them are at rule scope, and hence numbered at the top level. Aliasing the
square brackets to C<$1> means that the next subpattern at the same level
(i.e. the C<< (<[A..E]>) >>) is numbered sequentially (i.e. C<$2>), etc.


=back

=head3 Scalar aliases applied to quantified constructs

=over

=item *

All of the above semantics apply equally to aliases which are bound to
quantified structures.

=item *

The only difference is that, if the aliased construct is a subrule or
subpattern, that quantified subrule or subpattern will have returned a
list of C<Match> objects (as described in L<Quantified subpattern
captures> and L<Repeated captures of the same subrule>).
So the corresponding array element or hash entry for the alias will
contain a referece to an array, instead of a single C<Match> object.

=item *

In other words, aliasing and quantification are completely orthogonal.
For example:

     if m/ mv $0:=<file>+ / {
         # <file>+ returns a list of Match objects,
         # so $0 contains a reference to an array of Match objects,
         # one for each successful call to <file>

         # $/<file> does not exist (it's pre-empted by the alias)
     }


     if m/ mv $<from>:=(\S+ \s+)* / {
         # Quantified subpattern returns a list of Match objects,
         # so $/<from> contains a reference to an array of Match
         # objects, one for each successful match of the subpattern

         # $0 does not exist (it's pre-empted by the alias)
     }

=item *

Note, however, that a set of quantified I<non-capturing> brackets always
returns a single C<Match> object which contains only the complete
substring that was matched by the full set of repetitions of the
brackets (as described in L<Named scalar aliases applied to 
non-capturing brackets>). For example:

     "coffee fifo fumble" ~~ m/ $<effs>:=[f <-[f]>**{1..2} \s*]+ /;

     say $<effs>;    # prints "fee fifo fum"


=back

=head3 Array aliasing

=over

=item *

An alias can also be specified using an array as the alias instead of scalar.
For example:

     m/ mv @<from>:=[(\S+) \s+]* <dir> /;

=item *

Using the C<< @<alias>:= >> notation instead of a C<< $<alias>:= >>
mandates that the corresponding hash entry or array element I<always>
receives a reference to an array of C<Match> objects, even if the
construct being aliased would normally return a single C<Match> object.
This is useful for creating consistent capture semantics across
structurally different alternations (by enforcing array captures in all
branches):

     m:w/ Mr?s? @<names>:=<ident> W\. @<names>:=<ident>
        | Mr?s? @<names>:=<ident>
        /;

     # Aliasing to @<names> means $/<names> is always
     # an array reference, so...

     say @{$/<names>};

=item *

For convenience and consistency, C<< @<key> >> can also be used outside a
regex, as a shorthand for C<< @{ $/<key> } >>. That is:

     m:w/ Mr?s? @<names>:=<ident> W\. @<names>:=<ident>
        | Mr?s? @<names>:=<ident>
        /;

     say @<names>;

=item *

If an array alias is applied to a quantified pair of non-capturing
brackets, it captures the substrings matched by each repetition of the
brackets into separate elements of the corresponding array. That is:

     m/ mv $<files>:=[ f.. \s* ]* /; # $/<files> assigned a single
                                     # Match object containing the
                                     # complete substring matched by
                                     # the full set of repetitions
                                     # of the non-capturing brackets

     m/ mv @<files>:=[ f.. \s* ]* /; # $/<files> assigned an array,
                                     # each element of which is a
                                     # C<Match> object containing
                                     # the substring matched by Nth
                                     # repetition of the non-
                                     # capturing bracket match

=item *

If an array alias is applied to a quantified pair of capturing parens
(i.e. to a subpattern), then the corresponding hash or array element is
assigned a list constructed by concatenating the array values of each
C<Match> object returned by one repetition of the subpattern. That is,
an array alias on a subpattern flattens and collects all nested
subpattern captures within the aliased subpattern. For example:

     if m:w/ $<pairs>:=( (\w+) \: (\N+) )+ / {
         # Scalar alias, so $/<pairs> is assigned an array
         # of Match objects, each of which has its own array
         # of two subcaptures...

         for @{$<pairs>} => $pair {
             say "Key: $pair[0]";
             say "Val: $pair[1]";
         }
     }


     if m:w/ @<pairs>:=( (\w+) \: (\N+) )+ / {
         # Array alias, so $/<pairs> is assigned an array
         # of Match objects, each of which is flattened out of
         # the two subcaptures within the subpattern

         for @{$<pairs>} => $key, $val {
             say "Key: $key";
             say "Val: $val";
         }
     }

=item *

Likewise, if an array alias is applied to a quantified subrule, then the
hash or array element corresponding to the alias is assigned a list
containing the array values of each C<Match> object returned by each
repetition of the subrule, all flattened into a single array:

     rule pair :w { (\w+) \: (\N+) \n }

     if m:w/ $<pairs>:=<pair>+ / {
         # Scalar alias, so $/<pairs> contains an array of
         # Match objects, each of which is the result of the
         # <pair> subrule call...

         for @{$<pairs>} => $pair {
             say "Key: $pair[0]";
             say "Val: $pair[1]";
         }
     }


     if m:w/ mv @<pairs>:=<pair>+ / {
         # Array alias, so $/<pairs> contains an array of
         # Match objects, all flattened down from the
         # nested arrays inside the Match objects returned
         # by each match of the <pair> subrule...

         for @{$<pairs>} => $key, $val {
             say "Key: $key";
             say "Val: $val";
         }
     }

=item *

In other words, an array alias is useful to flatten into a single array
any nested captures that might occur within a quantified subpattern or subrule.
Whereas a scalar alias is useful to preserve within a top-level array
the internal structure of each repetition.

=item *

It is also possible to use a numbered variable as an array alias.
The semantics are exactly as described above, with the sole difference
being that the resulting array of C<Match> objects is assigned into the
appropriate element of the rule's match array, rather than to a key of
its match hash. For example:

     if m/ mv  \s+  @0:=((\w+) \s+)+  $1:=((\W+) (\s*))* / {
         #          |                 |
         #          |                 |
         #          |                  \_ Scalar alias, so $1 gets an
         #          |                     array, with each element
         #          |                     a Match object containing
         #          |                     the two nested captures
         #          |
         #           \___ Array alias, so $0 gets a flattened array of
         #                just the (\w+) captures from each repetition

         @from     = @{$0};      # Flattened list

         $to_str   = $1[0][0];   # Nested elems of
         $to_gap   = $1[0][1];   #    unflattened list
     }

=item *

Note again that, outside a rule, C<@0> is simply a shorthand for
C<@{$0}>, so the first assignment above could also have been written:

         @from = @0;


=back

=head3 Hash aliasing

=over

=item *

An alias can also be specified using a hash as the alias variable,
instead of a scalar or an array. For example:

     m/ mv %<location>:=( (<ident>) \: (\N+) )+ /;

=item *

A hash alias causes the correponding hash or array element in the
current scope's C<Match> object to be assigned a (nested) hash reference
(rather than an array reference or a single C<Match> object).

=item *

If a hash alias is applied to a subrule or subpattern then the first nested
numeric capture becomes the key of each hash entry and any remaining numeric
captures become the values (in an array if there is more than one),

=item *

As with array aliases it is also possible to use a numbered variable as
a hash alias. Once again, the only difference is where the resulting
C<Match> object is stored:

     rule one_to_many {  (\w+) \: (\S+) (\S+) (\S+) }

     if m:w/ %0:=<one_to_many>+ / {
         # $/[0] contains a hash, in which each key is provided by
         # the first subcapture within C<one_to_many>, and each
         # value is a reference to an  array containing the
         # subrule's second, third, and fourth, etc. subcaptures...

         for %{$/[0]} -> $pair {
             say "One:  $pair.key";
             say "Many: { @{$pair.value} }";
         }
     }

=item *

Outside the rule, C<%0> is a shortcut for C<%{$0}>:

         for %0 => $pair {
             say "One:  $pair.key";
             say "Many: { @{$pair.value} }";
         }


=back

=head3 External aliasing

=over

=item *

Instead of using internal aliases like:

     m/ mv  @<files>:=<ident>+  $<dir>:=<ident> /

the name of an ordinary variable can be used as an "external alias", like so:

     m/ mv  @files:=<ident>+  $dir:=<ident> /

=item *

In this case, the behaviour of each alias is exactly as described in the
previous sections, except that the resulting capture(s) are bound
directly (but still hypothetically) to the variables of the specified
name that exist in the scope in which the rule declared.


=back

=head2 Capturing from repeated matches

=over

=item *

When an entire rule is successfully matched with repetitions
(specified via the C<:x> or C<:g> flag) or overlaps (specified via the
C<:ov> or C<:ex> flag), it will usually produce a series
of distinct matches.

=item *

A successful match under any of these flags still returns a single
C<Match> object in C<$/>. However, the values of this match object are
slightly different from those provided by a non-repeated match:

=over

=item *

The boolean value of C<$/> after such matches is true or false, depending on
whether the pattern matched.

=item *

The string value is the substring from the start of the first match to
the end of the last match (I<including> any intervening parts of the
string that the rule skipped over in order to find later matches).

=item *

There are no array contents or hash entries.

=back

For example:

     if $text ~~ m:w:g/ (\S+:) <rocks> / {
         say 'Full match context is: [$/]';
     }

=item *

The list of individual match objects corresponding to each separate
match is also available, via the C<.matches> method. For example:

     if $text ~~ m:w:g/ (\S+:) <rocks> / {
         say "Matched { +$/.matches } times";

         for $/.matches -> $m {
             say "Match between $m.from() and $m.to()";
             say 'Right on, dude!' if $m[0] eq 'Perl';
             say "Rocks like $m<rocks>";
         }
     }

=back

=head2 C<:keepall>

=over

=item *

All rules remember everything if C<:keepall> is in effect
anywhere in the outer dynamic scope.  In this case everything inside
the angles is used as part of the key.  Suppose the earlier example
parsed whitespace:

     / <key> <?ws> <'=>'> <?ws> <value> { %hash{$<key>} = $<value> } /

The two instances of C<< <?ws> >> above would store an array of two
values accessible as C<< @<?ws> >>.  It would also store the literal
match into C<< $<'=\>'> >>.  Just to make sure nothing is forgotten,
under C<:keepall> any text or whitespace not otherwise remembered is
attached as an extra property on the subsequent node. (The name of
that property is "C<pretext>".)

=back

=head1 Grammars

=over

=item *

Your private C<ident> rule shouldn't clobber someone else's
C<ident> rule.  So some mechanism is needed to confine rules to a namespace.

=item *

If subs are the model for rules, then modules/classes are the obvious
model for aggregating them.  Such collections of rules are generally
known as "grammars".

=item *

Just as a class can collect named actions together:

     class Identity {
         method name { "Name = $.name" }
         method age  { "Age  = $.age"  }
         method addr { "Addr = $.addr" }

         method desc {
             print &.name(), "\n",
                   &.age(),  "\n",
                   &.addr(), "\n";
         }

         # etc.
     }

so too a grammar can collect a set of named rules together:

     grammar Identity {
         rule name :w { Name = (\N+) }
         rule age  :w { Age  = (\d+) }
         rule addr :w { Addr = (\N+) }
         rule desc {
             <name> \n
             <age>  \n
             <addr> \n
         }

         # etc.
     }

=item *

Like classes, grammars can inherit:

     grammar Letter {
         rule text     { <greet> <body> <close> }

         rule greet :w { [Hi|Hey|Yo] $to:=(\S+?) , $$}

         rule body     { <line>+ }

         rule close :w { Later dude, $from:=(.+) }

         # etc.
     }

     grammar FormalLetter is Letter {

         rule greet :w { Dear $to:=(\S+?) , $$}

         rule close :w { Yours sincerely, $from:=(.+) }

     }

=item *

Just like the methods of a class, the rule definitions of a grammar are
inherited (and polymorphic!). So there's no need to respecify C<body>,
C<line>, etc.

=item *

Perl 6 will come with at least one grammar predefined:

     grammar Perl {    # Perl's own grammar

         rule prog { <statement>* }

         rule statement { <decl>
                   | <loop>
                   | <label> [<cond>|<sideff>|;]
         }

         rule decl { <sub> | <class> | <use> }

         # etc. etc. etc.
     }

=item *

Hence:

     given $source_code {
         $parsetree = m:keepall/<Perl.prog>/;
     }

=back

=head1 Syntactic categories

For writing your own backslash and assertion rules or macros, you may
use the following syntactic categories:

     rule rxbackslash:<w> { ... }    # define your own \w and \W
     rule rxassertion:<*> { ... }    # define your own <*stuff>
     macro rxmetachar:<,> { ... }    # define a new metacharacter
     macro rxmodinternal:<x> { ... } # define your own /:x() stuff/
     macro rxmodexternal:<x> { ... } # define your own m:x()/stuff/

As with any such syntactic shenanigans, the declaration must be visible in
the lexical scope to have any effect.  It's possible the internal/external
distinction is just a trait, and that some of those things are subs
or methods rather than rules or macros.  (The numeric rxmods are recognized
by fallback macros defined with an empty operator name.)

=head1 Pragmas

The C<rx> pragma may be used to control various aspects of regex
compilation and usage not otherwise provided for.

=head1 Transliteration

=over

=item *

The C<tr///> quote-like operator now also has a method form called
C<trans()>.  Its argument is a list of pairs.  You can use anything that
produces a pair list:

     $str.trans( %mapping.pairs.sort );

Use the .= form to do a translation in place:

     $str.=trans( %mapping.pairs.sort );

=item *

The two sides of the any pair can be strings interpreted as C<tr///> would:

     $str.=trans( 'A..C' => 'a..c', 'XYZ' => 'xyz' );

As a degenerate case, each side can be individual characters:

     $str.=trans( 'A'=>'a', 'B'=>'b', 'C'=>'c' );

=item *

The two sides of each pair may also be array references:

     $str.=trans( ['A'..'C'] => ['a'..'c'], <X Y Z> => <x y z> );

=item *

The array version can map one-or-more characters to one-or-more
characters:

     $str.=trans( [' ',      '<',    '>',    '&'    ] =>
                  ['&nbsp;', '&lt;', '&gt;', '&amp;' ]);


In the case that more than one sequence of input characters matches,
the longest one wins.  In the case of two identical sequences the
first in order wins.

There are also method forms of C<m//> and C<s///>:

     $str.match(//);
     $str.subst(//, "replacement")
     $str.subst(//, {"replacement"})
     $str.=subst(//, "replacement")
     $str.=subst(//, {"replacement"})

=back

=head1 Matching against non-strings

=over

=item *

Anything that can be tied to a string can be matched against a
rule. This feature is particularly useful with input streams:

     my $stream is from($fh);       # tie scalar to filehandle

     # and later...

     $stream ~~ m/pattern/;         # match from stream

An array can be matched against a rule.  The special C<< <,> >>
rule matches the boundary between elements.  If the array elements
are strings, they are concatenated virtually into a single logical
string.  If the array elements are tokens or other such objects, the
objects must provide appropriate methods for the kinds of rules to
match against.  It is an assertion error to match a string matching
assertion against an object that doesn't provide a string view.
However, pure token objects can be parsed as long as the match rule
restricts itself to assertions like:

     <.isa(Dog)>
     <.does(Bark)>
     <.can('scratch')>

It is permissible to mix tokens and strings in an array as long as they're
in different elements.  You may not embed objects in strings, however.

To match against each element of an array, use a hyper operator:

     @arrayÂ».match($rule)

=back

