       return nil
     }
   }
+
+  /// Classify an anchor character.
+  ///
+  /// If in a custom character class:
+  /// Anchor -> <none>
+  ///
+  /// Otherwise:
+  /// Anchor -> '^' | '$' | '\b' | '\B' | '\A' | '\Z' | '\z' | '\G' | '\K' |
+  ///           '\y' | '\Y'
   private static func classifyAsAnchor(
     _ t: Character, fromEscape escaped: Bool, inCustomCharClass: Bool
   ) -> Anchor? {

extension Lexer {
  private static func classifyAsSpecialCharEscape(
    _ t: Character, inCustomCharClass: Bool
  ) -> Token.SpecialCharacterEscape? {

    return mc
  }

  /// Classify a given terminal character.
  ///
  /// Terminal -> Anchor | MetaChar | SpecialCharEscape | Character
  ///
  /// If .ignoreWhitespace:
  /// ' ' -> Trivia
  ///
  static func classifyTerminal(
    _ t: Character,
    fromEscape escaped: Bool,
