non - regex not

How to match, but not capture, part of a regex? (4)

By far the simplest (works for python) is '123-(apple|banana)-?456'.

I have a list of strings. Some of them are of the form 123-...456. The variable portion "..." may be:

  • the string "apple" followed by a hyphen, e.g. 123-apple-456
  • the string "banana" followed by a hyphen, e.g. 123-banana-456
  • a blank string, e.g. 123-456 (note there's only one hyphen)

Any word other than "apple" or "banana" is invalid.

For these three cases, I would like to match "apple", "banana", and "", respectively. Note that I never want capture the hyphen, but I always want to match it. If the string is not of the form 123-...456 as described above, then there is no match at all.

How do I write a regular expression to do this? Assume I have a flavor that allows lookahead, lookbehind, lookaround, and non-capturing groups.

The key observation here is that when you have either "apple" or "banana", you must also have the trailing hyphen, but you don't want to match it. And when you're matching the blank string, you must not have the trailing hyphen. A regex that encapsulates this assertion will be the right one, I think.

I have modified one of the answers (by @op1ekun):


The reason is that the answer from @op1ekun also matches "123-apple456", without the hyphen after apple.

Try this:




That will match apple, banana, or a blank string, and following it there will be a 0 or 1 hyphens. I was wrong about not having a need for a capturing group. Silly me.