Validates an Indian EPF / PF account number.
Format: SS/RO/XXXXXXX/000/XXXXXXX (e.g. MH/BAN/0012345/000/0001234).
SS/RO/XXXXXXX/000/XXXXXXX
MH/BAN/0012345/000/0001234
TextFormField(validator: v.epf)
String? epf(String? v) => EpfValidator(_msg).validate(v);