Exclusive Or in Regular Expression

Question

Looking for a bit of regex help. I\'d like to design an expression that matches a string with \"foo\" OR \"bar\", but not both \"foo\" AND \"b

Answer 1

If your regex language supports it, use negative lookaround:

(?

This will match "foo" or "bar" that is not immediately preceded or followed by "foo" or "bar", which I think is what you wanted.

It's not clear from your question or examples if the string you're trying to match can contain other tokens: "foocuzbar". If so, this pattern won't work.

Here are the results of your test cases ("true" means the pattern was found in the input):

foo: true
bar: true
foofoo: false
barfoo: false
foobarfoo: false
barbar: false
barfoofoo: false