Lookbehind in Regex searches

Regex's more esoteric features are easy to miss as they make sense to me only in a context that applies to my work. I've certainly read about lookahead and lookbehind before but until I needed it today and was pointed to look at this functionality in the Regex engine it just didn't sink in. Lookbehind allows matching or not matching of a string before the the string you are interested in. It effectively allows you to look for values that don't have x before another match which is actually quite common.