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From: PerlFAQ Server on 6 Jan 2010 06:00
This is an excerpt from the latest version perlfaq6.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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6.9: How can I quote a variable to use in a regex?

The Perl parser will expand $variable and @variable references in
regular expressions unless the delimiter is a single quote. Remember,
too, that the right-hand side of a "s///" substitution is considered a
double-quoted string (see perlop for more details). Remember also that
any regex special characters will be acted on unless you precede the
substitution with \Q. Here's an example:

$string = "Placido P. Octopus";
$regex = "P.";

$string =~ s/$regex/Polyp/;
# $string is now "Polypacido P. Octopus"

Because "." is special in regular expressions, and can match any single
character, the regex "P." here has matched the <Pl> in the original
string.

To escape the special meaning of ".", we use "\Q":

$string = "Placido P. Octopus";
$regex = "P.";

$string =~ s/\Q$regex/Polyp/;
# $string is now "Placido Polyp Octopus"

The use of "\Q" causes the <.> in the regex to be treated as a regular
character, so that "P." matches a "P" followed by a dot.



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