A regex filter is a cybersecurity tool that uses regular expressions (regex) to match specific patterns within text or code. By defining these patterns, security systems can identify, allow, or block certain strings, such as malicious code, suspicious URLs, or unauthorized data. Regex filters are widely used in firewalls, intrusion detection systems, email filters, and web applications to enhance security by automating the detection of threats. However, poorly designed regex filters can lead to false positives or negatives, so careful crafting and testing are essential. Overall, regex filters provide a flexible and powerful method for pattern-based filtering in cybersecurity.