String negation using regular expressions

regex lookahead negative-lookahead negation
Paul Bevis picture Paul Bevis · Jul 20, 2009 · Viewed 56.5k times · Source

Is it possible to do string negation in regular expressions? I need to match all strings that do not contain the string "..". I know you can use ^[^\.]*$ to match all strings that do not contain "." but I need to match more than one character. I know I could simply match a string containing ".." and then negate the return value of the match to achieve the same result but I just wondered if it was possible.

Answer

chaos picture chaos · Jul 20, 2009

You can use negative lookaheads:

^(?!.*\.\.).*$

That causes the expression to not match if it can find a sequence of two periods anywhere in the string.

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