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.