Why does .*(?!Integration)Test match FooIntegrationTest?

Question

I'm trying to write a regex to run all unit tests but not run integration tests. Unit tests are named FooTest, integration tests are named BarIntegrationTest, with "Foo" and "Bar" being variables. I found this article on how to do it and I have solved my problem. But, its solution is to use this regex:

(.(?!Integration))*Test

I don't understand why this regex doesn't suffice:

.*(?!Integration)Test

When I tried that second regex, my Integration tests were still run.

Answer 1

You're using a negative look ahead, but you want a negative look behind:

.*(?<!Integration)Test

Your regex is asserting that "Test" is not "Integration", which of course is always true.

Answer 2

.*(?!Integration)Test

This involves .* eating up the whole string and then backtracking to match Test.For exmaple if test string is Integration test .* eats up the whole string and then applies the looahead which passes as after Test there is no integration. Once .* backtracks to match t from Test it again applies lookahead but fails so proceeds and matches Test in similar way applying the lookahead at each step.