Regular Expressions
Open Group's
Single UNIX Specification Version 4
[login required; no cost to register] webpage provides a
description
of regular expressions in UNIX.
Books about Regular Expressions:
Mastering Regular Expressions, 3rd Edition
by Jeffrey E. F. Friedl (O'Reilly, 2006)
Regular Expressions Cookbook, 2nd Edition
by Jan Goyvaerts and Steven Levithan (O'Reilly, 2012)
Introducing Regular Expressions
by Michael Fitzgerald (O'Reilly, 2012)