pos: Int,
      limit: Int,
    ): Int {
      if (limit - pos < 2) return -1

      val c0 = input[pos]
      if ((c0 < 'a' || c0 > 'z') && (c0 < 'A' || c0 > 'Z')) return -1 // Not a scheme start char.

      characters@ for (i in pos + 1 until limit) {
        return when (input[i]) {
          // Scheme character. Keep going.
          in 'a'..'z', in 'A'..'Z', in '0'..'9', '+', '-', '.' -> continue@characters