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.

A writeup on regular expressions connecting the math. and the CS views to regular expressions.

An introduction from a University course.

Good overview of regular expression operators, although some features described might not be available in your implementation.

Downloadable reference with examples, by Miloslav Nic.

A tutorial with many examples which can be downloaded for off-line use.

Specification for regular expressions -- massive document, somewhat technical.