Regular expression combining match with negative lookahead

Question

I am looking for a single regular expression that achieves the following:

• only match words containing [a-z0-9-_] (so /^[a-z0-9-_]+$/i)
• don't match the word "test" (so /^(?!test).*$/i)

Both requirements are easy to implement on their own. But How would a regular expression look like that combines both requirements in a single expression?

Answer 1

You may just inline your matching logic into the second regex, i.e. use:

^(?!test$)[a-z0-9_-]+$

Demo

Note carefully the negative lookahead (?!test$). When placed at the very start of the pattern, this asserts that the match is not the exact word test. However, 1test and testing should be allowed, as they are not exactly test.

Also note that the character class I have used is slightly different than your version. We should place - at the very end of the class, because 9-_ will actually be interpreted as all characters in between 9 and underscore, which is probably not what you want.