Regular expressions. If you want an in-depth book try Mastering Regular Expressions by Friedl. Or for a quick guide, http://regular-expressions.info along with http://rubular.com/

Regex will not only give you new skills for programming (no more splitting strings in weird ways and doing complex indexOf/substr operations), it will help you day-to-day in rummaging through file systems and your code.

And if you already know regex, then I'd recommend Map Reduce Streaming. The Streaming variety is all based on standard in/out and very straight forward. Amazaon Elastic Map-Reduce could be a nice way to test things.