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
You need to use lookahead as some of the other responders have said, but the lookahead has to account for other characters between its target word and the current match position. For example: (?=.*word1)(?=.*word2)(?=.*word3) The .* in the first lookahead lets it match however many characters it needs to before it gets to “word1”. Then … Read more