There's no backslash interpretation going on within single-quote strings
which means there's no need to escape the backslash. Since this was going
on within single-quote strings the $exclude variable ended up having 2
backslashes (`\\`) instead of the intended single backslash. This meant
that the regex that was built up was incorrect. For example prometheus'
exclude contents before and after this change are:
✕: \(/_\|examples\|Godeps\|testdata\\|documentation/prometheus-mixin\)
✓: \(/_\|examples\|Godeps\|testdata\|documentation/prometheus-mixin\)