Perl match regex variable \Q - regex

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 (i.e. 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 :)