GNU awk User's Guide - Regular Expressions
   Documentation of the Regular Expression syntax used in GNU Awk.
   http://www.gnu.org/software/gawk/manual/html_node/Regexp.html

MSDN .NET Regular expressions
   MSDN documentation on .NET regular expressions.
   http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpgenref/html/cpconRegularExpressionsLanguageElements.asp

Mastering Regular Expressions (Second Edition)
   Book by Jeffrey Friedl. Includes table of contents, full index, an errata sheet.
   http://regex.info/

Posix Basic Regular Expression
   A description of both basic Posix Regular Expression syntax, the programming interface and some advanced features that are provided in the implementation of POSIX.2 regular expressions in the GNU C Library.
   http://chaos4.phy.ohiou.edu/~thomas/ref/info/rx/Top.html

Programming in Emacs Lisp - Regular Expression Searches
   Documentation of the Regular Expression syntax used in Emacs Lisp.
   http://www.gnu.org/software/emacs/emacs-lisp-intro/html_node/Regexp-Search.html

RE Tool
   Java based cross platform tool to learn Regular Expressions by "Trial & Error". Comes with Regular Expressions reference.
   http://www.indiwiz.com/products/retool/index.php

Regular Expression HOWTO
   Detailed HowTo documentation about regular expressions usage in combination with the Phyton programming/scripting language.
   http://www.amk.ca/python/howto/regex/regex.html

Regular Expression Quick Reference
   Single sheet quick reference card for regular expressions.
   http://www.night-ray.com/regex.pdf

Regular Expressions
   A brief and general introduction.
   http://www.ipgp.jussieu.fr/~choi/LGM/UNIX/regex.html

Regular Expressions - Single Unix Specification
   Specification for regular expressions -- massive document, somewhat technical.
   http://www.opengroup.org/onlinepubs/007908799/xbd/re.html

Regular Expressions Reference
   Downloadable reference with examples, by Miloslav Nic.
   http://www.zvon.org/other/reReference/Output/

Regular Expressions Tutorial
   A tutorial and reference for many popular regular expression flavors used by programming languages like Perl, PHP, Java, C# and VB.NET as well as a description of general tools like PowerGREP and EditPad.
   http://www.regular-expressions.info/

Regular Expressions Tutorial at Zvon
   A tutorial with many examples which can be downloaded for off-line use.
   http://www.zvon.org/other/PerlTutorial/Books/Book1/

Regular Expressions in ne (Nice Editor)
   A description of the regular expression support in ne (Nice Editor).
   http://www.emerson.emory.edu/services/editors/ne/Regular_Expressions.html

Regular Expressions: An Introduction
   An essential guide to one of the most useful tools in the web development: regular expressions.
   http://www.digitalamit.com/article/regular_expression.phtml

Regular expressions
   An introduction from a University course.
   http://www.seas.upenn.edu/~cit596/notes/dave/regexp0.html

Seeing With C - Regular Expressions
   A writeup on regular expressions connecting the math. and the CS views to regular expressions.
   http://www.seeingwithc.org/topic7html.html

Unicode Regular Expression Guidelines
   Unicode Technical Report #18: This document describes guidelines for how to adapt regular expression engines to use Unicode.
   http://www.unicode.org/unicode/reports/tr18/

Using Regular Expressions
   Good overview of regular expression operators, although some features described might not be available in your implementation.
   http://www.landings.com/_landings/pages/regex.html

grep - Regular Expressions
   Documentation of the Regular Expression syntax used in GNU grep.
   http://www.gnu.org/software/grep/doc/grep_7.html