       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 nil
    }
  }

  /// Classify a meta-character.
  ///
  /// In a custom character class:
  /// MetaChar -> '[' | ']' | '-' | '^'
  ///
  /// Otherwise:
  /// MetaChar -> '[' | '*' | '?' | '|' | '(' | ')' | '.' | ':'
  ///
  private static func classifyAsMetaChar(
    _ t: Character, inCustomCharClass: Bool
  ) -> Token.MetaCharacter? {
