A Tao of Regular Expressions
Yes, another regular expression tutorial. However, this one actually applies regular expression usage to command line tools like vi and grep. Extra bonus points to it for covering my personal favorite: the vi substitution command.
Most examples are presented as vi substitution commands or as grep file search commands, but they are representative examples and the concepts can be applied in the use of tools such as sed, awk, perl and other programs that support regular expressions.