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
I think I got it. First, as I mentioned in one of the comments, (?<=(?<A>.)(?<-A>.)) never matches. But then I thought, what about (?<=(?<-A>.)(?<A>.))? It does match! And how about (?<=(?<A>.)(?<A>.))? Matched against “12”, A is captures “1”, and if we look at the Captures collection, it is {“2”, “1”} – first two, then one … Read more
They are called lookarounds; they allow you to assert if a pattern matches or not, without actually making the match. There are 4 basic lookarounds: Positive lookarounds: see if we CAN match the pattern… (?=pattern) – … to the right of current position (look ahead) (?<=pattern) – … to the left of current position (look … Read more
Examples Given the string foobarbarfoo: bar(?=bar) finds the 1st bar (“bar” which has “bar” after it) bar(?!bar) finds the 2nd bar (“bar” which does not have “bar” after it) (?<=foo)bar finds the 1st bar (“bar” which has “foo” before it) (?<!foo)bar finds the 2nd bar (“bar” which does not have “foo” before it) You can … Read more