Dirty Secrets of the Perl Regex Engine: Slides from a talk the author presented at TPC and YAPC. Covers the /o flag and re 'eval'.
How Regexes Work: An article about how to use regular expressions in Perl by M-J Dominus.
MakeRegex: The Perl-module by Hakan Kjellerstrand composes a regex-expression from a list of words.
Perl FAQ : Regular Expressions: Pattern matching and regular expressions.
Perl Regex: An introduction covering basic concepts.
Perl Regular Expression Matching is NP-Complete: A discussion of the complexity of regular expression matching algorithms in Perl.
Perl Regular Expression Quick Reference: One-page document containing tabulated summaries of common codes, modifiers, and special characters. [PDF]
Perl Regular Expression Tutorial: A tutorial that shows how to use Regular Expressions in Perl.
perl.perl6.language.regex: Archive of discussion relating to regular expressions in Perl 6.
PERL5 Regular Expression Description: Detailed description written by Tom Christiansen in response to a question posted to comp.lang.perl.misc.
Perldoc.com - perlre: Describes the syntax of regular expressions in Perl.
Perldoc.com - perlretut: A tutorial for regular expressions in Perl.
perlretut: Basic tutorial, distributed as part of the core documentation.
Regular Expressions: A description of the patterns used in Perl for pattern matching.