boost regex tolerates non-escaped special characters, and escaped
non-special characters. Standardize on stricter syntax, where
special characters must be escaped, and non-special characters must
not.
I was hitting a stack size issue with the previous regular expression used
to match filesystem lines, so I decided to rewrite it. Admittedly I don't
know entirely if this has better performance, but it seems to work well
and additionally isn't too greedy with matching like the previous one.
