Overview
The Regular Expression Tester provides a real-time environment for writing, testing, and debugging regex patterns against sample text. All matches are highlighted inline in the text, match details are listed with their index positions and captured groups, and a library of 20 common regex patterns (email, URL, phone, date, IP address, etc.) is available for instant loading.
How to Use
Enter a regex pattern in the Pattern field (without surrounding slashes) and flags (g, i, m, s) in the Flags field. Type or paste test text in the large textarea. Matches highlight instantly in yellow. The match list below shows each match, its start/end position in the string, and any captured groups. Click any entry in the Common Patterns library to load that pattern automatically. Errors in the pattern (e.g., unclosed parentheses) are shown with a red border and error message.
Background & Context
Regular expressions were invented by mathematician Stephen Cole Kleene in the 1950s to describe formal languages. They were popularized in computing by Ken Thompson, who implemented them in the Unix text editor ed and the grep utility in 1973. The name ‘grep’ comes from the ed command g/re/p (globally search a Regular Expression and Print). Today regex engines are built into every major programming language and text editor. PCRE (Perl Compatible Regular Expressions) is the most widely used regex standard, implemented in PHP, Python, Java, Ruby, and many others.
