OK, the OP has persuaded me that while the other question mentioned has an overlapping topic, the fact that the forbidden string is forbidden at all locations, not just as a prefix, complicates things enough to require a separate answer, at least for the XSD 1.0 case. (I started to add this answer as an … Read more
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
You can use this (see on rubular.com): ^(?=[1-4]{4}$)(?!.*(.).*\1).*$ The first assertion ensures that it’s ^[1-4]{4}$, the second assertion is a negative lookahead that ensures that you can’t match .*(.).*\1, i.e. a repeated character. The first assertion is “cheaper”, so you want to do that first. References regular-expressions.info/Lookarounds and Backreferences Related questions How does the regular … 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
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