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This is an excerpt from the latest version perlfaq6.pod, which
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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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