How To Write Regex Example . A tool to generate simple regular expressions from sample text. A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern. How to Write Regular Expressions for Google Analytics Goals Boxcar from boxcarmarketing.com Regular expression, or regex or regexp in short, is extremely and amazingly powerful in searching and manipulating text strings, particularly in processing text files. A negative lookbehind does exactly the opposite, and the syntax is: However, using r prefix makes \ treat as a normal character.