CODEWITHSUNDEEP
regexregex-lookaroundsnegation
Paul Bevis
Paul Bevis

Reputation: 851

String negation using regular expressions

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.

Upvotes: 37

Views: 57289

Answers (3)

Greg Bacon
Greg Bacon

Reputation: 139711

If your regex engine does not support negative lookahead, then express what you want as arbitrarily many repetitions of either non-dot or dot followed by non-dot. (Another way to think of this is if you don’t see a dot, then you’re happy, but if you do see a dot, the next character must be not-dot.) Remember to accept the string consisting of a dot and nothing else.

As a regex, the above is

^([^.]|\.[^.])*\.?$

Note that this pattern matches the empty string because it does not contain dot-dot.

Upvotes: 1

Tim Pietzcker
Tim Pietzcker

Reputation: 336478

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

will only match if there are no two consecutive dots anywhere in the string.

Upvotes: 5

chaos
chaos

Reputation: 124365

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.

Upvotes: 43

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