What is the meaning of qr// in perl

Question

I am completely new to perl and trying to design a lexer where I have come across:

my @token_def =
 (
        [Whitespace => qr{\s+},     1],
        [Comment    => qr{#.*\n?$}m,   1],
  );

and even after going through multiple sites I did not understand the meaning.

http://perldoc.perl.org/functions/qr.html – Mat May 07 '15 at 06:17 — Mat, May 07 '15 at 06:17

score 21 · Answer 1 · edited May 07 '15 at 08:44

21

qr// is one of the quote-like operators that apply to pattern matching and related activities.

From perldoc:

This operator quotes (and possibly compiles) its STRING as a regular expression. STRING is interpolated the same way as PATTERN in m/PATTERN/. If ' is used as the delimiter, no interpolation is done.

From modern_perl:

The qr// operator creates first-class regexes. Interpolate them into the match operator to use them:

my $hat = qr/hat/;
say 'Found a hat!' if $name =~ /$hat/;

... or combine multiple regex objects into complex patterns:

my $hat   = qr/hat/;
my $field = qr/field/;

say 'Found a hat in a field!'
if $name =~ /$hat$field/;

like( $name, qr/$hat$field/,
            'Found a hat in a field!' );

edited May 07 '15 at 08:44

RobEarl

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answered May 07 '15 at 06:54

serenesat

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How is this different from `m/`? – Lou Aug 20 '20 at 08:22
2

@Lou - `qr` effectively compiles the regular expression before it is run, whereas `m` 'compiles' it on the fly. – Lee Goddard Oct 20 '20 at 11:18
maybe needless to say, but to use the `like` function you need to load the `test::more` module, as `like` is not part of the core perl functions – Haf Linger Nov 18 '22 at 09:57

score 5 · Answer 2 · edited May 07 '15 at 06:57

5

qr// is documented in perlop in the "Regexp Quote-Like Operators" section.

Just like qq"..." aka "..." allows you to construct a string, qr/.../ allows you to construct a regular expression.

$s = "abc";     # Creates a string and assigns it to $s
$s = qq"abc";   # Same as above. 
print("$s\n");

$re = qr/abc/;   # Creates a compiled regex pattern and assigns it to $x
print "match\n" if $s =~ /$re/;

The quoting rules for qr/.../ are very similar to qq"..."'s. The only difference is that \ followed by a non-word character are passed through unchanged.

edited May 07 '15 at 06:57

mpapec

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answered May 07 '15 at 06:30

ikegami

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After using `$re = qr/abc/`, it seems like you can just do `$s =~ $re` without the forward-slashes, since the forward-slashes are effectively compiling the regex and you have have already done that with `qr`. – CJ7 Oct 23 '20 at 06:04
@CJ7 yup, just like you can do `$s =~ "abc"`. In the absence of a qr//, m//, s/// or tr/// operator, a match operator (m//) is implied by the binding operator. But no, `/$re/` doesn't recompile the regex. – ikegami Oct 23 '20 at 06:13

What is the meaning of qr// in perl

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