In another post palindrome using regular expressions I asked about another ways to write catch a palindrome but my real interest is seeing examples of real world usage of extended regular expression. So if you have examples where you used any of the following constructs, I would appreciate seeing them. 
(?=pattern) - zero-width positive look-ahead
(?!pattern) - zero-width negative look-ahead
(?<=pattern) - zero-width positive look-behind
(?<!pattern) - zero-width negative look-behind
(?{ code }) - zero-width assertion with embedded code
(??{ code }) - postponed subexpression
(?>pattern) - "independent" subexpression
(?(condition)yes-pattern|no-pattern) - conditional expression
(?(condition)yes-pattern)

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In reply to real world extended regex examples by szabgab

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