[Fwirt]

Lexing is handled by one of Mitchell's other projects, Scintillua. You'll find the source for all the built-in lexers in there. https://github.com/orbitalquark/scintillua

The documentation for Scintillua also gets pulled into Textadept's API documentation as a dependency, so the syntax is also explained there. It's basically a bridge between Scintilla's native lexing and LPeg.