flowchart LR
  Start[Start]
  Stop[Stop]
  Start --> Digit([<tt>0</tt>..<tt>9</tt>])
  Digit --> Stop
  Digit --> NextDigit([<tt>0</tt>..<tt>9</tt> / <tt>_</tt>])
  NextDigit --> NextDigit
  NextDigit --> Stop

  style Start fill:#44E
  style Stop fill:#E44