Checking for patterns and validating the user inputs using Regular Expression
Regular expressions is a powerful object of VBScript to use in any string matching and particularly in pattern matching. This is highly important to maintain data uniformity and prevents any malicious code or characters to enter to the system other than the required patterns.
If user inputs are not validated the attackers can use this to get entry inside the protected area and this is known as injection attack. You can read more on this security issues here and here.
For example we don't want any one to use any character inside the field specified for entering zip or pin code inside a form. Same way for a phone number we may allow hyphens (- ) and numbers but not characters. User id should contain only number and characters and it should be minimum 3 and maximum 8 char length. We can check one valid email address by checking the pattern used.