Regex Tester
Test, debug, and explain regular expressions with real-time matching, group capturing, and replacement.
Enter a pattern and test string to see results
JWT Decoder & Parser
Decode and inspect JSON Web Token payloads →
All regex testing happens locally in your browser. Your test strings and patterns are never sent to any server, cached, or stored. Full support for all standard JavaScript regex features including lookaheads, backreferences, and named groups.
A regular expression (regex) is a sequence of characters that defines a search pattern, used for string matching, validation, extraction, and replacement. Our free Regex Tester lets you build, test, and debug regular expressions in real-time with instant match highlighting and detailed group information. All processing is done client-side with zero data uploads.
What Is a Regular Expression?
A regular expression (regex or regexp) is a
formal grammar for matching patterns in text. Originating from mathematical automata theory,
regular expressions are now a standard feature in nearly every programming language and text
editor. A regex pattern describes a set of strings using metacharacters (.*+?[](){}^$|) combined with literal characters. Modern regex engines support advanced constructs like
lookahead/lookbehind assertions, named capture groups, backreferences, and atomic groups.
How to Use the Regex Tester
- Enter your pattern — Type your regular
expression into the pattern input between the
/delimiters. - Set flags — Toggle regex flags like
g(global),i(case-insensitive), andm(multiline) by clicking the flag buttons. - Provide test text — Paste or type the string you want to test against in the test string textarea.
- View matches instantly — Matches are highlighted in real-time within the test string and detailed in the results panel below.
- Switch to Replace mode — Enter a replacement
string with
$1,$&, etc. to test find-and-replace operations.
Key Features
- Real-Time Highlighting
Matches are highlighted instantly within the test string as you type.
- Capture Group Details
View each match with its index and all captured groups displayed clearly.
- Match & Replace Modes
Toggle between pure matching and find-and-replace with capture group references.
- All Standard Flags
Support for g, i, m, s, u, and y flags — clickable toggle buttons.
- Error Feedback
Clear error messages for invalid regex patterns or syntax errors.
- 100% Private
All processing runs client-side. Nothing leaves your browser.
Common Use Cases
- Input Validation: Test patterns for email addresses, phone numbers, URLs, credit card numbers, and other structured data.
- Log Parsing: Extract structured data from server logs, CSV files, or any text with consistent formatting.
- Code Search & Refactoring: Find code patterns across large codebases and perform targeted search-and-replace operations.
- Data Scraping: Extract specific content from HTML, JSON, or plain text documents using carefully crafted patterns.
Frequently Asked Questions
What regex engine does this tool use?
RegExp engine (ECMAScript specification). It supports all standard features including lookaheads
((?=), (?!)),
lookbehinds ((?<=), (?<!)),
named capture groups ((?<name>...)), and Unicode property escapes (\p{}) with the u flag.What do the regex flags do?
g(global) — Find all matches, not just the first.i(case-insensitive) — Ignore case when matching.m(multiline) —^and$match line boundaries.s(dotAll) —.matches newline characters.u(unicode) — Enable Unicode property escapes and correct Unicode handling.y(sticky) — Match only from the last index position.
How do I use capture groups in replacement?
$1, $2, etc. for
numerical groups, $& for the full match, $` for text before the match, and $' for text after. Named groups are referenced as $<name>.Why is my regex not matching anything?
g flag for multiple matches, incorrect escaping of special characters, or pattern anchors (^ / $) that don't
align with your input. Use the error feedback to spot syntax problems and check that
your flags are set appropriately for your use case.