regex - String negation using regular expressions

Question

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 1

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.