A regular expression is a sequence of characters that defines a search pattern for text. To match a string of 123ABCDEFGHJ, you need to use the following regular expression: \d{3}[A-Z]{9}. This expression means that the string must start with three digits (\d{3}), followed by nine uppercase letters ([A-Z]{9}). This expression will match any string that has the same format as 123ABCDEFGHJ. References = User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Regular Expressions [Cisco Secure Email Gateway] - Cisco