=begin pod

=TITLE Quoting Constructs

=SUBTITLE Writing strings, word lists, and regexes in Perl 6

=head1 The Q Lang

Strings are usually represented in Perl 6 code using some form of quoting
construct. The most minimalistic of these is C<Q>, usable via the shortcut
C<｢…｣>, or via C<Q> followed by any pair of delimiters surrounding your
text. Most of the time, though, the most you'll need is C<'…'> or C<"…">,
described in more detail in the following sections.

=head2 X<Literal strings: Q|quote,Q;quote,｢ ｣>

=for code :allow<B>
B<Q[>A literal stringB<]>
B<｢>More plainly.B<｣>
B<Q ^>Almost any non-word character can be a delimiter!B<^>

Delimiters can be nested, but in the plain C<Q> form, backslash escapes
aren't allowed.  In other words, basic C<Q> strings are as literal as
possible.

=for code :allow<B>
Q<Make sure you B«<»matchB«>» opening and closing delimiters>
Q{This is still a closing curly brace → B<\>}

These examples produce:

    A literal string
    More plainly.
    Almost any non-word character can be a delimiter!
    Make sure you <match> opening and closing delimiters
    This is still a closing curly brace → \

The other quote forms add to this basic functionality:

=head2 X<Escaping: q|quote,q;quote,' '>

=for code :allow<B>
B<'>Very plainB<'>
B<q[>This backB<\s>lash staysB<]>
B<q[>This backB<\\>slash staysB<]> # Identical output
B<q{>This is not a closing culy brace → B<\}>, but this is → B<}>
B<Q :q $>There are no backslashes here, only lots of B<\$>B<\$>B<\$>!B<$>
B<'>(Just kidding. ThereB<\'>s no money in that string)B<'>
B<'>No $interpolation {here}!B<'>
B<Q:q#>Just a literal "\n" hereB<#>

The C<q> form allows for escaping characters that would otherwise end the
string using a backslash. The backslash itself can be escaped, too, as in
the third example above. The usual form is C<'…'> or C<q> followed by a
delimiter, but it's also available as an adverb on C<Q>, as in the fifth and
last example above.

These examples produce:

    Very plain
    This back\slash stays
    This back\slash stays
    This is not a closing curly brace → } but this is →
    There are no backslashes here, only lots of $$$!
    (Just kidding. There's no money in that string)
    No $interpolation {here}!
    Just a literal "\n" here

=head2 X<Interpolation: qq|quote,qq;quote," ">

=for code :allow<B L>
my $color = 'blue';
L<say> B<">My favorite color is B<$color>!B<">

    My favorite color is blue!

The C<qq> form -- usually written using double quotes -- allows for
interpolation of variables, i.e., variables can be written within the string
so that the content of the variable is inserted into the string. It is also
possible to escape variables within a C<qq>-quoted string:

=for code :allow<B>
say B<">The B<\>$color variable contains the value '$color'B<">;

    The $color variable contains the value 'blue'

Another feature of C<qq> is the ability to interpolate Perl 6 code from
within the string, using curly braces:

=for code :allow<B L>
my ($x, $y, $z) = 4, 3.5, 3;
say "This room is B<$x> m by B<$y> m by B<$z> m."
say "Therefore its volume should be B<{ $x * $y * $z }> m³!"

    This room is 4 m by 3.5 m by 3 m.
    Therefore its volume should be 42 m³!

By default, only variables with the C<$> sigil are interpolated normally.
This way, when you write C<"documentation@perl6.org">, you aren't
interpolating the C<@perl6> variable. If that's what you want to do, append
a C<[]> to the variable name:

=for code :allow<B>
my @neighbors = "Felix", "Danielle", "Lucinda";
say "@neighborsB<[]> and I try our best to coexist peacefully."

    Felix Danielle Lucinda and I try our best to coexist peacefully.

Often a method call is more appropriate.  These are allowed within C<qq>
quotes as long as they have parentheses after the call. Thus the following
code will work:

=for code :allow<B L>
say "@neighborsB<.L<join>(', ')> and I try our best to coexist peacefully."

    Felix, Danielle, Lucinda and I try our best to coexist peacefully.

However, C<"@example.com"> produces C<@example.com>.

=head2 X«Word quoting: qw|quote,qw;quote,< >»

=for code :allow<B L>
B«<»a b cB«>» L<eqv> ('a', 'b', 'c')
B<qw|>! @ # $ % ^ & * \| < > B<|> eqv '! @ # $ % ^ & | < >'.words
B<Q:w {> [ ] \{ \} B<}> eqv ('[', ']', '{', '}')

The C<:w> form, usually written C«<…>» or C<qw>, splits the string into
"words".  In this context, words are defined as sequences of non-whitespace
characters separated by whitespace.  The C<q:w> and C<qw> forms inherit the
interpolation and escape semantics of the C<q> and single quote string
delimiters, whereas C<Qw> and C<Q:w> inherit the non-escaping semantics of
the C<Q> quoter.

This form is used in preference to using many quotation marks and commas for
lists of strings. For example, where you could write:

    my @directions = 'left', 'right,', 'up', 'down';

It's easier to write and to read this:

    my @directions = <left right up down>;

=head2 X<Word quoting with interpolation: qqw|quote,qqw;quote,« »>

The C<qw> form of word quoting doesn't interpolate variables:

    my $a = 42; say qw{$a b c};  # $a b c

Thus, if you wish for variables to be interpolated within the quoted string,
you need to use the C<qqw> variant:

    my $a = 42;
    my @list = qqw{$a b c};
    say @list;                # 42 b c

or equivalently:

    my $a = 42;
    my @list = «$a b c»;
    say @list;                # 42 b c

=head2 X<Shell quoting: qx|quote,qx>

To run a string as an external program, not only is it possible to pass the
string to the C<shell> or C<run> functions but one can also perform shell
quoting in a similar manner to the backticks a.k.a. C<qx> in Perl 5.  There
are some subtleties to consider, however.  The backticks are no longer used
for shell quoting in Perl 6 and the C<qx> quotes I<don't> interpolate Perl
variables.  Thus

    my $world = "there";
    say qx{echo "hello $world"}

prints simply C<hello>.  Nevertheless, if you have declared an environment
variable before calling C<perl6>, this will be available within C<qx>, for
instance

    WORLD="there" perl6
    > say qx{echo "hello $WORLD"}

will now print C<hello there>.

The result of calling C<qx> is returned, so this information can be assigned
to a variable for later use:

    my $output = qx{echo "hello!"};
    say $output;    # hello!

See also L<shell|/routine/shell>, L<run|/routine/run> and L<Proc::Async> for
other ways to execute external commands.

=head2 X<Shell quoting with interpolation: qqx|quote,qqx>

If one wishes to use the content of a Perl variable within an external
command, then the C<qqx> shell quoting construct should be used (this
corresponds to Perl 5's C<qx>):

    my $world = "there";
    say qqx{echo "hello $world"};  # hello there

Again, the output of the external command can be kept in a variable:

    my $word = "cool";
    my $option = "-i";
    my $file = "/usr/share/dict/words";
    my $output = qqx{grep $option $word $file};
    # runs the command: grep -i cool /usr/share/dict/words
    say $output;      # Cooley␤Cooley's␤Coolidge␤Coolidge's␤cool␤ ...

See also L<shell|/routine/shell> and L<run|/routine/run> for other ways to
execute external commands.

=head2 Heredocs: :to

A convenient way to write multi-line string literals are I<heredocs>, which
let you choose the delimiter yourself:

=begin code
say q:to/END/;
Here is
some multi-line
string
END
=end code

The contents of the heredoc only begin on the next line, so you can (and
should) finish the line.

=begin code
my $escaped = my-escaping-function(q:to/TERMINATOR/, language => 'html');
Here are the contents of the heredoc.
Potentially multiple lines.
TERMINATOR
=end code

If the terminator is indented, that amount of indention is removed from the
string literals.  Therefore this heredoc

=begin code
say q:to/END/;
    Here is
    some multi line
        string
    END
=end code

produces this output:

=begin code
Here is
some multi line
    string


=end code

Heredocs include the newline from before the terminator.

=head1 Regexes

For information about quoting as applied in regexes see the L<regular
expression documentation|/language/regexes>.

=end pod
