roho has asked for the wisdom of the Perl Monks concerning the following question:

My environment: Perl 5.24.1 on Windows 10.

The following SSCCE uses the "qr" operator to quote a string as a regular expression. The non-capturing part of the regex curiously has a "caret" inserted between the question mark and the colon. I've looked and can find no explanation or example of what the "caret" does in the regex produced by "qr". The "caret" does NOT (thankfully) anchor the regex at the beginning of the line, so what does it do? 

#!/usr/bin/perl
use strict;
use warnings;

my $text  = 'abcd';
my $regex = qr/$text/;

print "\n\$text = |$text|    \$regex = |$regex|\n\n";
__END__

Output:

$text = |abcd|    $regex = |(?^:abcd)|

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"It's not how hard you work, it's how much you get done."

Replies are listed 'Best First'.
Re: Purpose of the "caret" character in "qr" regex output
by jwkrahn (Abbot) on Jul 31, 2025 at 01:47 UTC 
    $ man perlre

    Starting in Perl 5.14, a "^" (caret or circumflex accent)
    immediately after the "?" is a shorthand equivalent to
    "d-imnsx".  Any positive flags (except "d") may follow
    the caret, so

        (?^x:foo)

    is equivalent to

        (?x-imns:foo)

    The caret tells Perl that this cluster doesn't inherit the
    flags of any surrounding pattern, but uses the system
    defaults ("d-imnsx"), modified by any flags specified.

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    Naked blocks are fun! -- Randal L. Schwartz, Perl hacker
[reply]
[d/l]
      Thank you jwkrahn.

      "It's not how hard you work, it's how much you get done."

[reply]
Re: Purpose of the "caret" character in "qr" regex output
by GrandFather (Saint) on Jul 31, 2025 at 01:59 UTC

    In (?<modifiers>:...) <modifiers> (if present) is some mixture of the 'imopsx' modifiers usually found at the end of a regex. '^' is a modifier in this context that means "use the system default modifiers 'd-imsx'". I'm guessing qr does this to get predictable behavior when $regex is embedded in a larger regex.

    Consider:

    use strict;
use warnings;

my $text  = 'Abcd';
my $match = 'abcd';
my $regex = qr/$match/;

print "No match: '$text', '$match'\n" if $text !~ /$regex/i;
print "/i match: '$text', '$match'\n" if $text =~ /$match/i;

    [download]

    Prints:

    No match: 'Abcd', 'abcd'
/i match: 'Abcd', 'abcd'

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    Optimising for fewest key strokes only makes sense transmitting to Pluto or beyond
[reply]
[d/l]
[select]
      Thank you GrandFather.

      "It's not how hard you work, it's how much you get done."

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