A regular expression (regex) is a sequence of characters that defines a search pattern for text. A regex can be used to extract custom properties from log messages or events in a SIEM platform. In this case, the regex that matches the phrase "File: Clean" exactly is ^File: Clean$. The ^ symbol indicates the beginning of the line and the $ symbol indicates the end of the line. The regex ensures that no other characters are before or after the phrase. References:
* Understanding Cisco Cybersecurity Operations Fundamentals (CBROPS) - Cisco, Module 5: Security Policies and Procedures, Lesson 5.3: Data and Event Analysis
* 200-201 CBROPS - Cisco, Exam Topics, 5.0 Security Policies and Procedures, 5.3 Analyze data as part of security monitoring activities
* Cisco Certified CyberOps Associate Overview - Cisco Learning Network, Videos, 5.3 Analyze data as part of security monitoring activities