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.