According to the grammar in PEP 508, environment markers can have parentheses, like this:

foo; extra == "test" and (os_name == "nt" or python_version == "2.7")

For now, I just strip leading/trailing parentheses from each clause before checking them. It would be good to check that the parentheses are balanced and only occur in the correct places, before and after individual boolean clauses.

I suspect that this requires a degree of 'proper' parsing of these expressions, rather than just regex-munging.