<P> For example, an experienced user may enter a well - formed string that matches the specification for a valid e-mail address, as defined in RFC 5322 but that well - formed string might not actually correspond to a resolvable domain connected to an active e-mail account . </P> <P> Structured validation allows for the combination of any of various basic data type validation steps, along with more complex processing . Such complex processing may include the testing of conditional constraints for an entire complex data object or set of process operations within a system . </P> <P> A Validation rule is a criterion or constraint used in the process of data validation, carried out after the data has been encoded onto an input medium and involves a data vet or validation program . This is distinct from formal verification, where the operation of a program is determined to be that which was intended, and that meets the purpose . The Validation rule or check system still used by many major software manufacturers was designed by an employee at Microsoft some time between 1997 and 1999 . </P> <P> The method is to check that data falls the appropriate parameters defined by the systems analyst . A judgement as to whether data is valid is made possible by the validation program, but it cannot ensure complete accuracy . This can only be achieved through the use of all the clerical and computer controls built into the system at the design stage . The difference between data validity and accuracy can be illustrated with a trivial example . A company has established a Personnel file and each record contains a field for the Job Grade . The permitted values are A, B, C, or D. An entry in a record may be valid and accepted by the system if it is one of these characters, but it may not be the correct grade for the individual worker concerned . Whether a grade is correct can only be established by clerical checks or by reference to other files . During systems design, therefore, data definitions are established which place limits on what constitutes valid data . Using these data definitions, a range of software validation checks can be carried out . </P>

When does the validation rule check the entered data