regex module: variable-width lookbehind In addition to the answer by HamZa, for any regex of any complexity in Python, I recommend using the outstanding regex module by Matthew Barnett. It supports infinite lookbehind—one of the few engines to do so, along with .NET and JGSoft. This allows you to do for instance: import regex if … 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
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