Add support for symbol classes.

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.