How do I escape meta-characters in a user-defined string?

[Kozz]

In the example below, the results are I found a match: "a" I found a match: "b" There was no match for "a+b" Now, the problem is that the + in the third key of the hash is being interpreted as a description of the regular expression, not as a literal character. If I have a large hash which contains these types of meta-characters in the keys, how do I escape those meta-characters to prevent them being interpreted?

#!/usr/bin/perl

my %hash=(
        'a',    '2',
        'b',    '5',
        'a+b',  '7',
        ); 

my $string='a+b';
my $key;

foreach $key (keys %hash){
  if($string=~/$key/){
    print "I found a match: \"$key\"\n";
  }else{
    print "There was no match for \"$key\"\n";
  }
}

Originally posted as a Categorized Question.

[btrott]

if($string=~/\Q$key\E/){

Quotes the metacharacters.