I’ve just today discovered that, inside a regex, $_ takes on the value of the string being matched against:

16:12 >perl -wE "my $string = 'abcde'; $_ = 42; say; $string =~ /(?{ s
+ay })/;"
42
abcde

16:12 >perl -v

This is perl 5, version 22, subversion 0 (v5.22.0) built for MSWin32-x
+64-multi-thread

[download]

This brings up two questions. First, is it documented anywhere? It’s not mentioned in the perlvar entry for $ARG.

Second, what is the rationale for this behaviour?

Here’s the background:

Following on from the recent thread Shorthand for /^\s*abc\s*$/ ?, I was experimenting with (??{ }) as an alternative means of parameterizing a regex without using a sub. This works:

use strict;
use warnings;

my %searches =
(
    bar  => " bar  \n\n",
    bar1 => " bar1  \nw\n",
    baz  => "baz",
    foo  => "  foo  c  \n",
    foo1 => " foo ",
);

my $x;
my $regex = qr{ ^ \s* (??{ $x }) \s* $ }x;

for (sort keys %searches)
{
    $x = $_;
    print "$_: ", $searches{$_} =~ $regex ? 'match' : 'no match', "\n"
+;
}

[download]

but the use of $x is clumsy. So I tried $_ instead:

my $regex = qr{ ^ \s* (??{ $_ }) \s* $ }x;

[download]

After some debugging using perl -Mre=debug ..., it finally dawned on me that within the regex, $_ no longer contains the current hash key, but has been reset to the current hash value (i.e., the string on the left of the =~) with whitespace removed.1

Which seems strange to me, as well as unnecessary. Is there a situation in which this implicit assignment is actually useful?

1Update: Sorry, it’s (??{ $_ }) that has whitespace removed, due to the /x modifier.

Thanks,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

In reply to Value of $_ inside a regex by Athanasius

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