regular expression - match word only once in line

regex regex-negation
user1135229 picture user1135229 · Jan 6, 2012 · Viewed 12.1k times · Source

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 want to match line 2 (contains 'hello' more than once)

Tried using negative look ahead look behind and what not... without any real success..

Answer

Kobi picture Kobi · Jan 6, 2012

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...

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