The answer to the question you ask, which is whether a larger class of languages than the regular languages can be recognised with regular expressions augmented by lookaround, is no. A proof is relatively straightforward, but an algorithm to translate a regular expression containing lookarounds into one without is messy. First: note that you can … Read more
A negative lookahead says, at this position, the following regex can not match. Let’s take a simplified example: a(?!b(?!c)) a Match: (?!b) succeeds ac Match: (?!b) succeeds ab No match: (?!b(?!c)) fails abe No match: (?!b(?!c)) fails abc Match: (?!b(?!c)) succeeds The last example is a double negation: it allows b followed by c. The … Read more
Use a non-consuming regular expression. The typical (i.e. Perl/Java) notation is: (?=expr) This means “match expr but after that continue matching at the original match-point.” You can do as many of these as you want, and this will be an “and.” Example: (?=match this expression)(?=match this too)(?=oh, and this) You can even add capture groups … Read more