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 simple option is this (using the multiline flag and not dot-all):

^(?!.*\bhello\b.*\bhello\b).*\bhello\b.*$

First, check you don't have 'hello' twice, and then check you have it at least once.
There are other ways to check for the same thing, but I think this one is pretty simple.

Of course, you can simple match for \bhello\b and count the number of matches...