How to obtain value of Perl regex match variable with index stored in another variable? - perl

Question

I have a subroutine that takes as input the a position in a string, and should return the word found at that position. For example:
use warnings;
use strict;
my $num=2;
my $val=getMatch($num);
sub getMatch {
my ($num)=#_;
my $str='a b c';
$str=~ /(\S+)\s(\S+)/;
my $res;
eval "$res=\$$num";
return $res
}
But this gives error:
Use of uninitialized value $res in concatenation (.) or string at ./p.pl line 16.
(I am trying to return $i where i is a value stored in another variable..)

Answer 1

I'd do:
my $num=2;
my $val=getMatch($num);
say $val;
sub getMatch {
my ($num)=#_;
my $str='a b c';
my #res = $str =~ /(\S+)\s(\S+)/;
return $res[$num-1];
}
Output:
b

Answer 2

You could use the #+ and #- special variables, documented in perlvar, like this:
sub getMatch {
my ($num)=#_;
my $str='a b c';
$str=~ /(\S+)\s(\S+)/;
return substr( $str, $-[$num], $+[$num] - $-[$num] );
}
print getMatch(1), "\n";
print getMatch(2), "\n";
Or you could adjust your regex like this:
sub getMatch {
my $num = shift() - 1;
my $str='a b c';
$str=~ /(?:\S+\s){$num}(\S+)/;
return $1;
}
print getMatch(1), "\n";
print getMatch(2), "\n";
...which has the advantage of producing only a single capture group.
Another option is to just split on space:
sub getMatch {
my ($num)=#_;
my $str='a b c';
return ( split /\s/, $str )[$num-1];
}
print getMatch(1), "\n";
print getMatch(2), "\n";
...but that last solution is more permissive as to what it will match; it doesn't explicitly require two or more non-space items separated by a space. If 3 were passed in, it would return 'c'.
This last one produces results similar to the split version but using a regex. I'd probably prefer the split because it's more straightforward, but I provide this just for edification:
sub getMatch {
my ($num)=#_;
my $str='a b c';
return ( $str =~ m/(\S+)(?=\s|$)/g )[$num-1];
}
print getMatch(1), "\n";
print getMatch(2), "\n";