(which it used to be) in a regex; Also added code to detect escaped
backslashes
Specifically, I replace an escaped backslash with a metacharacter, then
replace it back later on. This prevents problems, like detecting whether
the opening bracket is escaped in '\\[a]'.
I added support for hex values (eg. \x0F), octal values (eg. \012) and
extended hex values (eg. \x{000F2A}). I also expanded the abilities of
character clsses, to include things like escaped characters (eg. [aefp\)])
and character ranges _inside_ inverted character classes (eg. [^\w] which is
functionally equivalent to [\W]).
I added a new function 'Compile' that calls shuntingYard and thompson. I also added
a new type 'Reg' that this function returns - it represents the starting state and contains
the number of capturing groups in the regex. I also rewrote shuntingYard and thompson
to return errors instead of panicking.
