Question

How to test if a capture is defined in Perl regexp matching?

#!/usr/bin/env perl
use v5.10;
my $str="foobar
barfoo";

while($str =~ m/(?:(f.*))|(?:(b.*))/g) {
    say "+ $1";
    say "- $2";
}

It outputs

+ foobar
- 
+ 
- barfoo

while what I want is

+ foobar
- barfoo

I.e. if $1 or $2 aren’t matched, it shouldn’t print.

Answer 1

The thing is you aren’t testing if it’s defined, so it’s going to print the +/- either way.

while($str =~ m/(?:(f.*))|(?:(b.*))/g) {
    say "+ $1" if defined($1);
    say "- $2" if defined($2);
}