It is important to validate each field until it is either valid or an error has been detected. The order of testing data is to first check for missing data. Then a syntax test can check the length of the data entered and check for proper class and composition. Only after the syntax is correct are the semantics, or meaning, of the data validated. This includes a range, reasonable, or value test, followed by a check digit test.<\/p>\n
GUI screens help to reduce the number of human input errors when they incorporate radio buttons, check boxes, and drop-down lists. When radio buttons are used, one should be set as the default, and the only way it would be unchecked is if the user clicks a different radio button. In the case of drop-down lists, the first choice should contain a message informing the user to change the list. If the first choice is still selected when the form is submitted, a message should inform the user to select a different option.<\/p>\n
Usually validating a single field is done with a series of IF . . . ELSE statements, but there are also pattern validation methods. Usually these patterns are found in the database design (as in Microsoft Access) but may be included in programming languages, such as Perl, JavaScript, and XMLschemas. The patterns are called regular expressions and contain symbols that represent the type of data that must be present in a field. Table shown below illustrates characters used in JavaScript regular expressions.<\/p>\n