Question

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How can I escape meta-characters when I interpolate a variable in Perl's match operator?

I am using the following regex to search for a string $word in the bigger string $referenceLine as follows :

$wordRefMatchCount =()= $referenceLine =~ /(?=\b$word\b)/g

The problem happens when my $word substring contains some (, etc. Because it takes it as a part of the regex rather than the string to match and gives the following error :

Unmatched ( in regex; marked by <-- HERE in 
m/( <-- HERE ?=\b( darsheel safary\b)/ 
at ./bleu.pl line 119, <REFERENCE> line 1.

Can somone please tell me a solution to this? I think If I could somehow get perl to understand that we want to look for the whole $word as it is without evaluating it, it might work out.

Answer 1

Use

$wordRefMatchCount =()= $referenceLine =~ /(?=\b\Q$word\E\b)/g

to tell the regex engine to treat every character in $word as a literal character.

\Q marks the start, \E marks the end of a literal string in Perl regex.

Alternatively, you could do

$quote_word = quotemeta($word);

and then use

$wordRefMatchCount =()= $referenceLine =~ /(?=\b$quote_word\b)/g

One more thing (taken up here from the comments where it’s harder to find:

Your regex fails in your example case because of the word boundary anchor \b. This anchor matches between a word character and a non-word character. It only makes sense if placed around actual words, i. e. \bbar\b to ensure that only bar is matched, not foobar or barbaric. If you put it around non-words (as in \b( darsheel safary\b) then it will cause the match to fail (unless there is a letter, digit or underscore right before the ().