Section 3 - Character Classes

Looking at the Twitter spec more closely, digits and _ characters are also allowed in the username portion of the handle. A simple Range will not be sufficient here, so we can turn to CharacterClass and it’s associated Spec types.

import ropes.core._
import ropes.core.Spec._

type Username      = Repeated[1, 15, CharacterClass[('a' - 'z') || ('A' - 'Z') || ('0' - '9') || ==['_']]]
type TwitterHandle = Literal['@'] Concat Username

To keep things concise, it uses symbolic definitions, so let’s at these in turn:

• 'x' - 'y' defines that any characters from x up to and including y are allowed
• ==['x'] defines that only character x is allowed
• a || b defines that characters matching the spec a or the spec b are allowed