The Core iOS Developer’s Cookbook, Fifth Edition

Recipe: Text-Entry Filtering

You might sometimes want to ensure that a user enters only a certain subset of characters. For example, you might want to create a numeric-only text field that does not handle letters. Although you can use predicates to test the final entry against a regular expression (the NSPredicate class’s MATCH operator supports regex values and is demonstrated in Recipe 6-10), for filtered data, it’s easier to check each new character as it is typed against a legal set.

A UITextField delegate can catch those characters as they are typed and decide whether to add each character to the active text field. The optional textField:shouldChangeCharactersInRange:replacementString: delegate method returns either YES, allowing the newly ...