Regex Tester
Test regular expressions with real-time matching Free, browser-based, no sign-up required.
What is Regex Tester?
Regular expressions (regex) are powerful pattern-matching sequences used to search, validate, and manipulate text in virtually every programming language and text editor. This free online Regex Tester lets you write a regex pattern, set flags (global, case-insensitive, multiline, dotAll, unicode), and instantly see all matches highlighted in your test string. It displays match details including capture groups, match indices, and total match count. You can also test find-and-replace operations with regex substitution. Everything runs entirely in your browser using the native JavaScript RegExp engine — no data is sent to any server.
How to Use
- Enter your regular expression pattern in the pattern field.
- Toggle the desired flags (g, i, m, s, u) using the checkboxes next to the pattern.
- Type or paste your test string into the textarea below.
- Matches are highlighted in real time in the output area.
- View match details (index, full match, captured groups) in the table below.
- Optionally enter a replacement string to preview regex substitution.
- Use preset buttons to quickly load common patterns like email, URL, or phone number with example text.
How It Works
The tool constructs a JavaScript RegExp object from your pattern and selected flags, then executes it against the test string using exec() in a loop (for global matches) or a single exec() call. Matches are extracted with their indices and capture groups. For the highlighted output, the tool escapes HTML entities in the original text and wraps matched portions in <mark> tags. The replace feature uses String.prototype.replace() with the constructed regex. Invalid patterns are caught and displayed as friendly error messages.
Common Use Cases
- Testing and debugging regular expressions before embedding them in application code
- Validating input patterns for forms — emails, phone numbers, dates, URLs, credit cards
- Extracting structured data from unstructured text using capture groups
- Learning regex syntax interactively with instant visual feedback and presets
- Building find-and-replace transformations for text processing scripts
- Verifying regex behavior across different flag combinations (case-insensitive, multiline, etc.)
- Prototyping data validation rules for backend or frontend form handlers
Frequently Asked Questions
Is my data safe?▼
Yes. All regex processing happens directly in your browser using JavaScript's built-in RegExp engine. Your test strings are never sent to any server, stored, or logged. You can verify this with your browser's network inspector.
Which regex flavor does this tool use?▼
This tool uses the JavaScript (ECMAScript) regex engine built into your browser. Most standard regex features — character classes, quantifiers, lookaheads, lookbehinds (in modern browsers), backreferences, and named groups — are fully supported.
What do the flags mean?▼
g (global) finds all matches instead of stopping at the first. i (case-insensitive) ignores letter case. m (multiline) makes ^ and $ match line boundaries. s (dotAll) makes the dot (.) match newline characters. u (unicode) enables full Unicode matching and proper surrogate pair handling.
Why is my regex causing the browser to hang?▼
Some patterns with nested quantifiers can cause catastrophic backtracking — exponential time complexity on certain inputs. For example, (a+)+b on a string of many 'a's without a 'b'. Simplify nested quantifiers or use atomic groups / possessive quantifiers where possible.
Can I use lookbehind assertions?▼
Lookbehind assertions (?<=...) and (?<!...) are supported in modern browsers (Chrome 62+, Firefox 78+, Safari 16.4+, Edge 79+). If your browser doesn't support them, you'll see a syntax error message.