In #pm's IRC tonight, Petruchio was looking for a way to use constants (defined via "use constant") in regex's. The no-frills way is to just assign a temporary variable with the constant value, but this leads to extra code. Here's a way to do it with no extra variables. Simply, you need to wrap it in the 'magic' code that forces any expression to be evaluated when it's used in a string (that is '@{[ expr ]}'), and then use the \Q and \E modifiers to tell perl to execute that part of the code before starting the regex.

Update: As MeowChow's pointed out to me, it's not necessary to \Q\E within the substitition part of s/// for this to work, as well as in m// strings as well; code snippet has been updated to reflect this. Mainly, if you do so, realize any special symbols ('.', '{', '}', etc) wil be quoted (eg '.' to '\.'), if you do include the \Q\E. 

use constant TEST => 3;
my $string = "123456";

$string =~ s/\Q@{[ TEST ]}\E/9/;
print $string, "\n";             # 129456

$string =~ s/2/@{[ TEST ]}/;
print $string, "\n";             # 139456

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In reply to Using constants inside regex by Masem

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