[IBR-003-OM] - Seller VATIN (IBT-031), Buyer VATIN (IBT-048) and Third party VATIN (BTOM-006) MUST be 12 characters: 'OM' followed by exactly 10 digits.
IBR-003-OM cac:AccountingSupplierParty/cac:Party/cac:PartyTaxScheme[cac:TaxScheme/cbc:ID='VAT']/cbc:CompanyID | cac:AccountingCustomerParty/cac:Party/cac:PartyTaxScheme[cac:TaxScheme/cbc:ID='VAT']/cbc:CompanyID | cac:AccountingSupplierParty/cac:Party/cac:AgentParty/cac:PartyTaxScheme[cac:TaxScheme/cbc:ID='VAT']/cbc:CompanyID matches($cleanID, '^OM[0-9]{10}$')