Question

When I do this

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $s = 'dfgdfg5 )';
my $a = '5 )';
my $b = '567';

$s =~ s/$a/$b/g;
print Dumper $s;

I get

Unmatched ) in regex; marked by <-- HERE in m/5 ) <-- HERE / at ./test.pl line 11.

The problem is that $a have a (.

How do I prevent the regex from failing?

Update

The string in $a do I get from a database query, so I can’t change it. Or would it be possible to make an $a2 where “something” searches for ) and replaces them with \)?

Answer 1

You need to escape it. Either manually by adding backslash in front of it, or by using quotemeta or the \Q sequence inside the regex:

$a = quotemeta($a);

Or

$s =~ /\Q$a/$b/g;

ETA: This is a good option if you want to match literal strings from a database query.

You should also be aware that it is not a good idea to use $a and $b as variables, since they will mask the predefined variables that are used with sort. E.g. sort { $a <=> $b } @foo.