While we could add a hyphen to the regular expression, it seems sloppy to use a regular expression to test if a known value is present.  What this test is doing is taking the existing known environment, attempting to create a new one, and proving that this new environment is useful to child processes.  Consider, in some pathological case, what if merge-env actually corrupted all of the existing environment entries?  So, to that end, I think this is a more meaningful approach, testing explicitly whether or not our new env is in fact just like our old one with some new values added.