Adding new escapes to regex module

Other regex implementations have escape sequences for horizontal 
whitespace (`\h` and `\H`) and vertical whitespace (`\v` and `\V`).

The regex module already supports `\h`, but I can't use `\v` because it 
represents `\0x0b', as it does in the re module.

Now that someone has asked for it, I'm trying to find a nice way of 
adding it, and I'm currently thinking that maybe I could use `\y` and 
`\Y` instead as they look a little like `\v` and `\V`, and, also, 
vertical whitespace is sort-of in the y-direction.

As far as I can tell, only ProgressSQL uses them, and, even then, it's 
for what everyone else writes as `\b` and `\B`.

I want the regex module to remain compatible with the re module, in case 
they get added there sometime in the future.

Opinions?