CODEWITHSUNDEEP
regexperl
Stefan
Stefan

Reputation: 631

How can I use a variable for a regex pattern without interpreting meta characters?

$text_to_search = "example text with [foo] and more";
$search_string = "[foo]";

if ($text_to_search =~ m/$search_string/)
    print "wee";

Please observe the above code. For some reason I would like to find the text "[foo]" in the $text_to_search variable and print "wee" if I find it. To do this I would have to ensure that the [ and ] is substituted with [ and ] to make Perl treat it as characters instead of operators.

How can I do this without having to first replace [ and ] with \[ and \] using a s/// expression?

Upvotes: 63

Views: 164600

Answers (3)

Dave Cross
Dave Cross

Reputation: 69274

Use \Q to autoescape any potentially problematic characters in your variable.

if($text_to_search =~ m/\Q$search_string/) print "wee";

Update: To clarify how this works...

The \Q will turn on "autoescaping" of special characters in the regex. That means that any characters which would otherwise have a special meaning inside the match operator (for example, *, ^ or [ and ]) will have a \ inserted before them so their special meaning is switched off.

The autoescaping is in effect until one of two situations occurs. Either a \E is found in the string or the end of the string is reached.

In my example above, there was no need to turn off the autoescaping, so I omitted the \E. If you need to use regex metacharacters later in the regex, then you'll need to use \E.

Upvotes: 82

Boris Ivanov
Boris Ivanov

Reputation: 4254

You can use quotemeta (\Q \E) if your Perl is version 5.16 or later, but if below you can simply avoid using a regular expression at all.

For example, by using the index command:

if (index($text_to_search, $search_string) > -1) {
    print "wee";
}

Upvotes: 25

Platinum Azure
Platinum Azure

Reputation: 46193

Use the quotemeta function:

$text_to_search = "example text with [foo] and more";
$search_string = quotemeta "[foo]";

print "wee" if ($text_to_search =~ /$search_string/);

Upvotes: 52

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