How do I escape metacharacters in a user-defined string?

Coyo has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (regular expressions)

I run the following script and get the following output: What am I doing wrong here?

- a confused canine.

bash-2.02$ perl
$formula = $bit = '(4+5)';
if( $formula =~ /$bit/ ) {
 print "yeah\n";
} else {
 print "Oh no!\n";
}
^D
Oh no!
[download]

Originally posted as a Categorized Question.

Comment on How do I escape metacharacters in a user-defined string? Download Code

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Re: How do I escape metacharacters in a user-defined string?⭐ by Ovid (Cardinal) on Jul 08, 2000 at 02:18 UTC
This is an easy mistake to make. I've made it myself. Notice that in $bit, you have parentheses '()' and a plus '+'. What your regex is doing is trying to find one or more fours followed by one five. It then, if successful, would capture this to $1. The easiest way to deal with the is to use the "quote" metacharacters (\Q and \E) in the regex. `$formula = $bit = '(4+5)'; if( $formula =~ /\Q$bit\E/ ) { print "yeah\n"; } else { print "Oh no!\n"; }` [download] This will work as you expect. However, you'll have to be careful. Here's a warning from perlop: You cannot include a literal $ or @ within a \Q sequence. An unescaped $ or @ interpolates the corresponding variable, while escaping will cause the literal string \$ to be inserted. You'll need to write something like m/\Quser\E\@\Qhost/.	[reply] [d/l]
Re: How do I escape metacharacters in a user-defined string? by Perlmage (Acolyte) on Jul 11, 2000 at 10:11 UTC
You could also use `quotemeta()`, which is how the \Q is implemented. Ie., $formula = '(4+5)'; $bit = quotemeta $formula; if ( $formula =~ /$bit/ ) { print "yeah\n"; } else { print "Oh no!\n"; } This will handle literal '$' and '@'.	[reply] [d/l]