Regex for matching something if it is not preceded by something else

With regex in Java, I want to write a regex that will match if and only if the pattern is not preceded by certain characters. For example:

String s = "foobar barbar beachbar crowbar bar ";

I want to match if bar is not preceded by foo. So the output would be:

barbar
beachbar
crowbar
bar

Answer

You want to use negative lookbehind like this:

w*(?<!foo)bar

Where (?<!x) means “only if it doesn’t have “x” before this point”.

See Regular Expressions – Lookaround for more information.

Edit: added the w* to capture the characters before (e.g. “beach”).