Given the code: 
#!/usr/bin/perl

use strict;
use warnings;

my $test_string = "This is not a waffle";

print "Match: $1\n" if $test_string =~ /(?:f)\1/;

[download]
I get the following error (Win2K, perl v5.8.6):
Reference to nonexistent group in regex; marked by <-- HERE in m/(?:f)\1 <-- HERE / at C:\Documents\foo.pl line 8. (where line 8 is the last line in the snippet)

Note that this is actually referring to the lack of capturing parens. If I change the last line to
print "Match: $1\n" if $test_string =~ /((?:f)\1)/;
there are no errors but also no match. Interesting, if you ask me.

In reply to Re^2: Regular Expressions by Nkuvu
in thread Regular Expressions by y8

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