mathParser.py :: remove \A from regex

- it is unnecessary as it resolves to 'A' on python 2.7.4 (when inside a character class)
- it breaks on python 2.7.3 [http://imgur.com/3ZN9eyu]

See: http://bugs.python.org/issue13899

Don't ask why I'm running python 2.7.3 :)