What is the purpose of REGEX function? By using this function we can build validation rule to enforces proper data format.
Example : Validates Account Billing Zip/Postal Code is in 5 digit format or 9 digit format if billing country is USA.
Steps to create validation rule:
Go to steps -> Build -> Customize -> Account -> Validation Rule and enter required fields and save validation rule.
Error condition formula for this example:
See the below screen for reference.
Testing above validation Rule:
To test this validation rule go to account tab and give Billing country = USA and Billig ZIP/Postal code = any number which is not equal to 5 or 9 digit and try to save account record, you will get error message. See below screen for reference.