Add support for symbol classes.

[RobertDurfee]

At the moment, no symbol classes are supported (e.g. \d, \w, \s, etc.). Support is pretty trivial to implement, but requires both front and back end support.

Workarounds:

• Symbol classes are merely a convenience. They can be easily implemented using symbol sets instead (e.g. [0-9] instead of \d) which are fully supported.