Look-behind regex by John D. Cook.
From the post:
Look-behind is one of those advanced/obscure regular expression features that I don’t use frequently enough to remember the syntax, but just frequently enough that I wish I could remember it.
Look-behind can be positive or negative. Look-behind says “match this position only if the preceding text matches (does not match) the following pattern.”
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I wish I had read this post before writing regular expressions to clean up over 4K of scanning results recently. I can think of several cases where this could have been helpful.
If you want to practice your regex writing skills, visit Stack Overflow and try your hand at recent regex questions. Or stroll through some of the older questions for tips/techniques.