Search Regex adds a powerful set of search and replace functions to WordPress that go beyond the standard searching capabilities, with full regex support.
I can't tell you why it didn't find any occurences. If the expressions matches then it will return results, otherwise it won't. This must mean your expression doesn't match.
I tried to look for all occurences of that are on a single line, that is
but not
or
So I used
but found no occurencesin posts, even if they exists. Any idea why?