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I've seen one which is a single text box with grey placeholder text that has hyphens ("12-34-56") yet does not accept hyphens, so you need to type in the format

• HPsquared 5y ago
  This is basically how Excel works when you use custom number formatting. You can set a number format to display numeric data in a specified way (e.g. "00-00-00" for a sort code, or "0000 0000 0000 0000" for a CC number). Excel will store the data as a number, and display it with the selected format, but you can't type it in with the spaces etc. - it needs to be input as just a number. I think this way of doing things is quite common.