This site does a masterful job of explaining
regular expressions in ruby, java, & perl:
http://www.regular-expressions.info

The ruby page is a good starting point:
http://www.regular-expressions.info/ruby.html

And there is a printed book. I plan to have a
copy on my bookshelf:
http://www.regular-expressions.info/print.html

-- 
Eric Armstrong
TreeLight Health, Citizens' Advisory
http://www.citizensAdvisory.org -- politics & economics
http://www.treelight.com/health -- nutrition & fitness
http://www.artima.com -- software design & development