Regex 'or' operator avoid repetition

Question

How can I use the or operator while not allowing repetition? In other words the regex:

(word1|word2|word3)+

will match wo

Answer 1

The lookahead solutions will not work in several cases, you can solve this properly, without lookarounds, by using a construct like this:

(?:(?(1)(?!))(word1)|(?(2)(?!))(word2)|(?(3)(?!))(word3))+

This works even if some words are substrings of others and will also work if you just want to find the matching substrings of a larger string (and not only match whole string).

Live demo.

It simply works by failing the alteration if it has been matched previously, done by (?(1)(?!)). (?(1)foo) is a conditional, and will match foo if group 1 has previously matched. (?!) always fails.