Add README
Use set rather than a class variable.

Class variables are baaad. Sinatra already has a way to
set up stuff like this.
Two spaces.

Ruby people use two spaces, not four.
Use do/end for multi-line blocks
Convert to modular app.

Leaking stuff globally is bad.
Don't use () with no arguments.
if not => unless
Use literal regexp pattern.

There's no reason to go through the object itself.

Also, you can interpolate, no reason to use strings.
better quote usage
pattern usage is superfluous
POST -> GET.

There's no reason to use POST here. It's better to
use GET.