Regular Expressions are a powerful pattern-matching language. To see what's possible once you learn Regular Expressions, read what a Perl
master has to say:
http://virtual.park.uga.edu/humcomp/perl/regexps.html
(Using Regular Expressions)
For help learning Regular Expressions in the vi text
editor see "Searching and Replacing text" in this tutorial:
http://www.mvblind.uni-linz.ac.at/linux/guide/node171.html
(Linux User Guide: Advanced vi Tutorial)
Here is a Java applet that lets you experiment with Regular Expressions
online:
http://www.rutgers.edu/~sgro/perltutor/regexp.html
(Java Perl Regular Expression evaluator)