regular expression - match word only once in line

Question

Case:

1. ehello goodbye hellot hello goodbye
2. ehello goodbye hello hello goodbye

I want to match line 1 (only has \'hello\' once!) DO NOT w

Answer 1

A generic regex would be:

^(?:\b(\w+)\b\W*(?!.*?\b\1\b))*\z

Altho it could be cleaner to invert the result of this match:

\b(\w+)\b(?=.*?\b\1\b)

This works by matching a word and capturing it, then making sure with a lookahead and a backreference that it does/not follow anywhere in the string.