Regular Expression Opposite

Question

Is it possible to write a regex that returns the converse of a desired result? Regexes are usually inclusive - finding matches. I want to be able to transform a regex into

Answer 1

You can invert the character set by writing a ^ at the start ([^…]). So the opposite expression of [ab] (match either a or b) is [^ab] (match neither a nor b).

But the more complex your expression gets, the more complex is the complementary expression too. An example:

You want to match the literal foo. An expression, that does match anything else but a string that contains foo would have to match either

1. any string that’s shorter than foo (^.{0,2}$), or
2. any three characters long string that’s not foo (^([^f]..|f[^o].|fo[^o])$), or
3. any longer string that does not contain foo.

All together this may work:

^[^fo]*(f+($|[^o]|o($|[^fo]*)))*$

But note: This does only apply to foo.