Regex Tester - Test Regular Expressions Online
Free online regex tester. Enter a pattern and see matches highlighted in real-time. No signup needed.
About This Tool
Regex Tester lets you write, test, and debug regular expressions with real-time matching and highlighting against your test strings. It supports all JavaScript regex flags (g, i, m, s, u, y) and displays match groups, indices, and captured subgroups. A must-have for developers building input validation, data extraction, or search-and-replace logic.
How It Works
Regular expressions are patterns compiled into finite automata — specifically, a Non-deterministic Finite Automaton (NFA) in most JavaScript engines. When you type a pattern, the engine compiles it into an internal state machine. The test string is then processed character by character; at each position the engine attempts to match the pattern starting from that index.
This tool uses JavaScript's built-in `RegExp` object and the `matchAll()` method to find every occurrence. Each match object contains the full match, all captured groups (parenthesized subpatterns), the start index, and the input string. Flags modify behavior: 'g' enables global matching, 'i' makes it case-insensitive, 'm' treats ^ and $ as line boundaries, 's' makes the dot match newlines, 'u' enables Unicode mode, and 'y' enforces sticky matching. The tool highlights all matches in the test string and displays a detailed breakdown of each match group and its position.
How to Use
- 1Enter your regular expression pattern in the Pattern field.
- 2Select flags (g, i, m, s, u) to modify matching behavior.
- 3Type or paste your test string in the text area below.
- 4Matches are highlighted in real-time with index and capture group details.
- 5Use presets for common patterns or the cheat sheet for syntax reference.
Frequently Asked Questions
What is a regex tester?
A regex tester is a tool that lets you write regular expressions and test them against sample text in real-time. It highlights matches, shows capture groups, and helps you debug patterns quickly.
What are regex flags?
Flags modify how the regex engine processes the pattern. Common flags: g (global - find all matches), i (case-insensitive), m (multiline - ^ and $ match line boundaries), s (dotAll - . matches newlines), u (unicode support).
Is my data safe?
Yes. All processing happens entirely in your browser. No data is sent to any server. Your regex patterns and test strings never leave your device.
What regex syntax is supported?
This tool uses JavaScript's built-in RegExp engine, supporting all standard ECMAScript regex features including lookaheads, lookbehinds, named groups, and Unicode properties.