make the regex test more thorough

it seems like the std::regex delivered up to gcc 4.8 has problems with
constructs such as ".+". (at least if they appear at the end of a
regex...)

(this caused an exception on the norne deck if I used g++ 4.8 instead of clang 3.3. The corresponding opm-core PR is not necessary to merge to fix this, but I would strongly recommend doing so anyway...)