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

You could use a negative look-ahead containing a back reference:

^(?:(word1|word2|word3)(?!.*\1))+$

where \1 refers to the match of the capture group (word1|word2|word3).

Note that this assumes word2 cannot be formed by appending characters to word1, and that word3 cannot be formed by appending characters to word1 or word2.

Answer 2

Byers' solution is too hard coded and gets quite cumbersome after the letters increases.. Why not simply have the regex look for duplicate match?

([^\d]+\d)+(?=.*\1)

If that matches, that match signifies that a repetition has been found in the pattern. If the match doesn't work you have a valid set of data.

Answer 3

You could use negative lookaheads:

^(?:word1(?!.*word1)|word2(?!.*word2)|word3(?!.*word3))+$

See it working online: rubular

Answer 4

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.