I'm sorry, but I don't really recommend Zaxo's double e solution for serious purposes, as two e modifiers will have to evaluate the code on the right side every time a match is found and the replacement has to be calculated. A double e switch should usually be avoioded, just like a string eval. (If this isn't clear, s/$pat/$rep/ee is about the same as s/$pat/eval($rep)/e, and here the eval has to be run every time the pattern matches the string.)

A better solution would be to pass the replacement as a subroutine, so that the code doesn't have to be compiled multiple times. The first e modifier doesn't have to do this, as the code that generates the replacement string is always the same, so it doesn't have to be recompiled every time a match is found. (Of course, if the replace text is a constant string, you wouldn't need any e's.)

(This is of course for the case of your apply_regex_ee, where the replacement has to be computed. If the replacement is a constant string, as in apply_regex_e, you don't even need a single e, although it doesn't make the solution any worse to have the e switch there.)

Here's my version of the code. I'd like to thank chester for the clean example code.

use strict;
use warnings;

my $test_data = 'This is some test data: $888...!';
my $pattern = qr{(\$?[0-9,]+)};

my $replacement = '<test>$1<\/test>';
my $first_test = replace_with_constant($test_data, $pattern, $replacem
+ent);

my $replacement_func = sub { "<test>" . $1 . "<\/test>"; };
# OR my $replacement_func = sub { "<test>$1<\/test>"; };
my $second_test = replace_with_dynamic($test_data, $pattern, $replacem
+ent_func);

print $first_test, "\n", $second_test, "\n";

sub replace_with_constant {
    my ($data, $pat, $rep) = @_;

    $data =~ s/$pat/$rep/;
    # OR $data =~ s/$pat/$rep/e;
    return $data;
}

sub replace_with_dynamic {
    my ($data, $pat, $rep) = @_;

    $data =~ s/$pat/&$rep()/e;
    return $data;
}
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Update 2009 april 1: this seems to be a common question, Regular expression "replace string interpolation" problem asks it again.