Question

Can anyone suggest a regex to match the underscore in the following examples:

test_test test[_test test_] 

But NOT match this:

test[_]test 

This is using the .Net Regular Expression library. I’m using this RegEx tester to check:

http://derekslager.com/blog/posts/2007/09/a-better-dotnet-regular-expression-tester.ashx

Answer 1

Try this:

_[^\]]|[^[]_ 

It consists of an alternation of _[^\]] (underscore and not ]) and [^[]_ (not [ and underscore).

Or if you want to use look-around assertions to really match just the underscore and not surrounding characters:

_(?=[^\]])|_(?<=[^[]_) 

This matches any underscore that is not followed by a ] ((?=[^\]]), positive look-ahead) or any underscore that is not preceded by a [ ((?<=[^[]_), negative look-behind). And this can be combined to:

_(?:(?=[^\]])|(?<=[^[]_))