Symbolic Regex Matcher, SRM, is a new open source .NET regular expression matching tool and match generator in the Microsoft Automata framework. It is based on the .NET regex parser in combination with a set based representation of character classes. The main feature of the tool is that the core matching algorithms are based on symbolic derivatives that support extended regular expression operations such as intersection and complement and also support a large set of commonly used features such as bounded loop quantifiers. SRM has been adopted in an internal tool at Microsoft that scans for credentials and other sensitive content in cloud service software. With the built-in .NET regex engine the tool was susceptible to catastrophic backtracking on files with long lines, such as minified JavaScript and SQL server seeding files. SRM’s linear matching complexity has helped address these issues, while maintaining compatibility for the large set of .NET regexes used in the application.