Regular Expressions - Single Unix Specification - Specification for regular expressions -- massive document, somewhat technical. - http://www.opengroup.org/onlinepubs/007908799/xbd/re.html
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
Regular expressions - An introduction from a University course. - http://www.seas.upenn.edu/~cit596/notes/dave/regexp0.html
Regular Expressions - A brief and general introduction. - http://www.ipgp.jussieu.fr/~choi/LGM/UNIX/regex.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
grep - Regular Expressions - Documentation of the Regular Expression syntax used in GNU grep. - http://www.gnu.org/software/grep/doc/grep_7.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
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/
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 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 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
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
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/
Regular Expressions Reference - Downloadable reference with examples, by Miloslav Nic. - http://www.zvon.org/other/reReference/Output/
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
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