If you want to know whether a regular expression matches, why not simply use the regular expression in boolean context? A regular expression can match a zero-width string, so testing $& won't always help. I would use something like the following:

my $regexp = 'hello.*world';

if( /$regexp/ ) {
    print "Your regexp '$regexp' matches '$_'\n";
    for my $match ( 0..$#- ) {
        print sprintf "Match %02d (%d, %d):\t[%s]\n", $match, $-[$matc
+h], $+[$match], substr( $_, $-[$match], $+[$match] );
    };
};
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But as you haven't told us what the overall problem is you're trying to solve, I might be wildly off the mark. Maybe read XY Problem and then restate the actual problem you're trying to solve. That might be easier to solve than removing the problem where your current approach becomes untractable.

Hi sm@sh,

"1"==1, "1" eq 1, and looks_like_number(1) and looks_like_number("1") are all true in Perl. The StackOverflow thread that clueless newbie linked to already contains some excellent answers - especially choroba and ikegami's answers explain the caveats of peeking under the hood. That's why I recommended looking for a different solution to your problem than figuring out difference between 1 and "1".

Regards,
-- Hauke D

Are you trying to determine whether a regex had capture groups or not?

Thanks - that's very interesting (@LAST_MATCH_START).

Through some code I am currently working on I discovered a potential problem here, there is a subtle but very important distinction between @- and @+, from the former:

One can use $#- to find the last matched subgroup in the last successful match. Contrast with $#+, the number of subgroups in the [last successful] regular expression.

$ perl -MData::Dump -e '"ya"=~/(x)?/; dd $#-, \@-, $#+, \@+'
(0, [0], 1, [0, undef])
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And updating my previous example:

"1" =~ /1/    or die;
print $#-?"yes\n":"no\n";  # -> no
"1" =~ /(1)?/ or die;
print $#-?"yes\n":"no\n";  # -> yes
"1" =~ /(2)?/ or die;
print $#-?"yes\n":"no\n";  # -> no !!
"1" =~ /(2)?/ or die;
print $#+?"yes\n":"no\n";  # -> yes
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