Supplier document number pattern

[sde1000]

Deliveries have a field for "document number". The intent is that this is the delivery note number or invoice number of the document that's being referred to when the delivery is entered.

Staff don't always pick a useful number to enter, though — we've had them pick things like account number (same for all documents from the supplier), VAT registration number, and so on.

Where suppliers have a consistent format for the number we actually want the staff to use (eg. "INV-....." or "SX......") it would be useful to be able to specify the appropriate pattern as a regexp in the suppliers table to use to validate the number entered for each delivery. Since staff are unlikely to be familiar with regular expressions, it would also be useful to have a free text description of the reference to be entered to be displayed as a hint.