Excluding Matches With Regular Expressions

October 23, 2005

Here's an interesting regex problem:

I seem to have stumbled upon a puzzle that evidently is not new, but for which no (simple) solution has yet been found. I am trying to find a way to exclude an entire word from a regular expression search. The regular expression should find and return everything EXCEPT the text string in the search expression.

For example, if the word fox was what I wanted to exclude, and the searched text was:

The quick brown fox jumped over the lazy dog.

... and I used a regular expression of [^"fox"] (which I know is incorrect) (why this doesn't work I don't understand; it would make life SO much easier), then the returned search results would be:

The quick brown jumped over the lazy dog.

Regular expressions are great at matching. It's easy to formulate a regex using what you want to match. Stating a regex in terms of what you don't want to match is a bit harder.

One easy way to exclude text from a match is negative lookbehind:

\w+\b(?<!\bfox)

But not all regex flavors support negative lookbehind. And those that do typically have severe restrictions on the lookbehind, eg, it must be a simple fixed-length expression. To avoid incompatibility, we can restate our solution using negative lookahead:

(?!fox\b)\b\w+

You can test this regex in the cool online JavaScript Regex evaluator. Unfortunately, JavaScript doesn't support negative lookbehind, so if you want to test that one, I recommend RegexBuddy. It's not free, but it's the best regex tool out there by far-- and it keeps getting better with every incremental release.