Creating a custom validation rule

CakePHP provides a handful of validation rules out of the box, which together covers the need for most applications. The following table lists the built-in validation rules (found in CakePHP's Validation class.)

Rule	Purpose
_alphaNumeric	Checks that the value contains only integers or letters.
_between	Checks that the string length of the value is within the specified range.
_blank	Succeeds if the value is empty, or consists of only spaces (whitespaces, tabs, newlines, and so on).
_boolean	Checks if value can be interpreted as a Boolean.
_cc	Validates a credit card number.
_comparison	Compares the value to a given value, using the specified operator.
_custom	Validates the value using a custom regular expression.
_date	Validates the value as a date, using the given format or regular expression.
_decimal	Succeeds if value is a valid decimal number.
_email	Validates an e-mail address.
_equalTo	Succeeds...