Perl match regex variable \Q

Question

I'm trying to match a regex in perl. The regex needs to be stored in a variable.

From this question I got \\Q to match regex in a variable.

$regex = "\\$[0-9] (\\+|\\*) [0-9]";
$str = "$2 * 2";

if ($str =~ /\Q$regex/) {    # regex is: \$[0-9] (\+|\*) [0-9]
    print "Expression found :)\n";
} else {
    print "Expression not found :(\n";
}

This matches fine in regexpal . It also works fine when I use the regex immediately without first putting it in $regex (ie without the \\Q ). What is the \\Q doing to mess up my regex?

Answer 1

The \\Q and \\E pair can be used to escape all non-word characters within a double-quoted string context. For instance

perl -E 'say "abc[\Q[..]\E]def"'

output

abc[\[\.\.\]]def

I wonder why you think you need it, as it prevents all regex metacharacters from having their special effect. For instance \\Q[0-9] will match exactly [0-9] instead of any single decimal digit

I would write your code like this. Note that I have changed double quotes to qr// when defining the pattern to create a compiled regex, and to single quotes when defining the target string to avoid Perl trying to interpolate built-in variable $2 into the string. You must always use strict and use warnings 'all' at the top of every Perl program you write

use strict;
use warnings 'all';

my $regex = qr/\$[0-9] [+*] [0-9]/;
my $str   = '$2 * 2';

if ( $str =~ $regex ) {
    print "Expression found :)\n";
}
else {
    print "Expression not found :(\n";
}

output

Expression found :)