The Regex Tester is a free browser-based tool for writing, testing, and debugging regular expressions with live match highlighting. Regular expressions are patterns that describe text: they can match email addresses, extract numbers, validate formats, and find complex patterns across multi-line strings. Testing regex without immediate visual feedback is slow and error-prone. This tester highlights every match in your test text as you type the pattern, shows the number of matches found, and displays capture group values for each match. It supports all standard JavaScript RegExp flags: g (global, find all matches), i (case-insensitive), m (multiline, where ^ and $ match line boundaries), and s (dotAll, where . matches newlines). The tool also shows a quick reference of common regex syntax to help when you forget the exact notation for a lookahead or a word boundary. For complete results, JSON Formatter can format and validate JSON data, Find & Replace can find and replace text patterns, and Text Diff Checker can compare two texts and highlight differences.
Regular expressions are a compact language for describing patterns in text, used for input validation, string parsing, log analysis, and search-and-replace in code editors. This tester highlights all matches in real time as you type, shows named capture groups, and reports match count and position. Flags (global, multiline, case-insensitive, dotAll) can be toggled individually. The engine uses the JavaScript RegExp implementation built into your browser. Some key patterns that are frequently useful: matches a word boundary (the position between a word character and a non-word character), which prevents partial matches inside longer words. (?:...) creates a non-capturing group when you need grouping for alternation but do not need to capture the value. (?=...) and (?!...) are lookahead assertions that match a position without consuming characters. Named capture groups (?<name>...) are supported in modern JavaScript and make patterns more readable. For email validation, a pragmatic pattern like [^@s]+@[^@s]+.[^@s]+ is more practical than the technically complete RFC 5322 email regex, which is hundreds of characters long. When debugging complex patterns, test with both matching and non-matching examples to verify the pattern correctly rejects invalid input, not just accepts valid input.