Regex Tester: Build and Debug Patterns Online
Regex Tester is an online tool for testing and debugging regular expressions. Type a pattern, paste your text, and every match is highlighted in real time as you type — no button to press, no code to write.
Whether you are validating email addresses, parsing log files, or extracting data from text, it helps you get a pattern right fast. Color-coded capture groups, a built-in cheatsheet, a replace preview, and ten ready-made patterns make it useful for developers, students, and anyone wrangling text.
How to Test a Regex
Enter your pattern
Type your regular expression in the Regular Expression field. The pattern is validated as you type, and any syntax error appears inline straight away.
Set the flags
Toggle the flag buttons to change how matching works: g (global), i (case insensitive), m (multiline), s (dotall), and u (unicode). Changes apply instantly.
Paste your test string
Type or paste text into the Test String area. Matches are highlighted live, with each capture group shown in its own color.
Review the matches
The Match Details panel lists every match with its position and capture-group values, while a badge shows the total match count. Open Replace mode to test substitutions or the Cheatsheet for quick syntax help.
Features
Live Matching
Matches update automatically as you edit the pattern or test string — no button to click, with debounced input for smooth performance.
In-Place Highlighting
Every match is highlighted directly inside your test text so you can see exactly what the pattern catches.
Color-Coded Groups
Each capture group gets a distinct color (up to five), covering both numbered groups and named groups.
Match Details Panel
See each match with its character positions plus numbered and named group values, alongside a live match-count badge.
Replace Mode
Test substitutions with $1, $2, or ${name} references and watch the result preview update in real time.
Copy Result
Copy the replace output to your clipboard with a single click once the substitution looks right.
Common Patterns Library
Load any of ten presets — Email, URL, IPv4, Phone, Date, Time, Hex Color, HTML Tag, Username, and Strong Password — each with sample text.
Built-In Cheatsheet
A collapsible reference covers six categories: character classes, quantifiers, anchors, groups and references, lookaround, and flags.
Toggle Flags
Switch global, case-insensitive, multiline, dotall, and unicode flags on or off with dedicated buttons that take effect immediately.
Inline Error Detection
Invalid pattern syntax is caught as you type, with a clear inline message explaining what went wrong.
Frequently Asked Questions
How do I test a regex online?
Type your pattern in the Regular Expression field, set any flags you need, then paste your text into the Test String area. Matches are highlighted live and listed in the Match Details panel — there is nothing to install or sign up for.
Which regex engine does this tool use?
It uses the JavaScript RegExp engine built into your browser, so it supports modern features such as lookahead, lookbehind, named groups, and Unicode property escapes.
Why does my regex work in the tester but not in my code?
This tool runs JavaScript regex. Other flavors — such as PCRE, Python, Java, PHP, or C# — share most syntax but differ on details like lookbehind support, named-group syntax, and escaping. Check that the same flags are set and watch for flavor-specific differences when you port a pattern.
What do the g, i, m, s, and u flags mean?
g (global) finds every match instead of stopping at the first. i ignores case. m (multiline) makes ^ and $ match the start and end of each line. s (dotall) lets . match newlines. u enables full Unicode matching.
How do capture groups work?
Parentheses () create capture groups, each grabbing part of the match. They are referenced as $1, $2, and so on in replacements. Named groups use (?<name>...) and are referenced as ${name}. Every group is shown in its own color in both the highlights and the Match Details panel.
What does "No match" mean?
It means the pattern matches no part of the current test string. Double-check the pattern syntax, make sure the right flags are set (for example i for case-insensitive matching), and confirm the test text actually contains what you expect.
Is my test data sent to a server?
No. All matching, replacing, and validation happen entirely in your browser — nothing is uploaded. You can confirm this by using the tool offline once the page has loaded.
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