Use raw string comparison instead of std::regex , as it is clearly not mature.

Summary: It seems to be hard to find a way to get regex to work properly across plateforms, so it seems like it is a better idea to just not use it.

Test Plan: Run the tests suite.

Reviewers: freetrader, sickpig, awemany, kyuupichan, CCulianu, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D291