Regex Tester

The Regex Tester tool allows users to test regular expressions against input text, highlighting matches in real time. Software developers, data analysts, and system administrators use it to validate patterns, extract data, and ensure code accuracy. It simplifies debugging and pattern creation, saving time and reducing errors.

S. Siddiqui

Edited by

S. SiddiquiFounder & Editor-in-Chief
Sources:MDN Web DocsW3CIETFUpdated May 2026
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2 matches found

#1hello@example.comat index 14
#2support@yourtoolsbase.comat index 35

What Is the Regular Expression Tester?

The Regular Expression Tester lets you write a regex pattern and test it against sample text in real time, with matching sections highlighted as you type. Regular expressions are a compact language for describing text patterns, and they come up constantly in web development for input validation, text search, data extraction, and string manipulation. Having an interactive tester where you can see matches and groups immediately makes the process of building and debugging a pattern much faster than dropping it straight into code.

JavaScript's regex engine follows the specification set out in ECMA-262. The MDN guide to regular expressions is the most comprehensive practical reference for the syntax, flags, and built-in methods available in browsers and Node.js. This tester runs the JavaScript engine directly, so the results here match exactly what you will get in a browser or Node environment.

How to Use the Regular Expression Tester

  1. Type your regex pattern into the pattern field. Do not include the surrounding slashes; those are JavaScript syntax, not part of the pattern itself.
  2. Set your flags: g for global (find all matches), i for case-insensitive, m for multiline, s for dotAll (make . match newlines), and u for full Unicode support.
  3. Paste your test string into the text area below. Matches are highlighted in real time as you adjust the pattern.
  4. The match details panel shows each match with its index position and any captured groups.
  5. Use the replace field to test a substitution and see what the result looks like before putting it in your code.

Technical Background

JavaScript regex patterns are compiled into a state machine that runs through the input string trying to find substrings that match the pattern. The engine supports most standard regex features: character classes ([a-z]), quantifiers (*, +, ?, {n,m}), anchors (^, $, ), groups (()), backreferences (\1), and lookaheads and lookbehinds.

On top of that, ES2018 added named capture groups ((?<name>...)), which let you pull out matched substrings by name rather than index. Given that named groups make regex patterns significantly more readable, they are worth using whenever you are writing a pattern for production code rather than a quick one-off search.

What is more, the u flag is important when working with strings that contain Unicode characters outside the Basic Multilingual Plane, such as emoji or historic scripts. Without it, the engine treats the string as a sequence of UTF-16 code units rather than Unicode code points, which can cause surprising behaviour with certain characters.

In practice, backtracking is the main source of performance issues in complex regex patterns. A pattern with multiple nested quantifiers on overlapping character classes can cause exponential backtracking on carefully crafted inputs, a vulnerability known as ReDoS (Regular Expression Denial of Service). If your regex will run against untrusted input, test it with inputs designed to trigger worst-case backtracking before deploying it.

Common Use Cases

  • Input validation: Email addresses, phone numbers, postal codes, and other structured inputs are commonly validated with regex. Build and test the pattern here before wiring it into a form validation library.
  • Data extraction: Pulling out specific fields from log lines, structured strings, or scraped text is a classic regex use case. Named capture groups make it straightforward to extract multiple fields in one pass.
  • Search and replace: Testing a replacement pattern against sample data before running it on real files avoids the risk of corrupting content with an incorrect substitution.
  • URL routing: Some server-side routing systems use regex to match URL patterns. Testing the pattern against example URLs here is faster than restarting the server after each change.

For encoding strings to be safe in URLs, the URL Encoder / Decoder handles percent-encoding. If you come across a Base64 string in the text you are parsing, the Base64 Encoder / Decoder lets you decode it without leaving the page.

Limitations to Know

JavaScript's regex engine does not support some features found in other languages. POSIX character classes like [:alpha:] are not available. Lookbehind support is present from ES2018 onwards but is not available in older browsers. Recursive patterns and conditional patterns, which are available in PCRE (used in PHP and Python), do not exist in the JavaScript engine.

That said, the tester is designed for JavaScript regex specifically. If you need to write a regex for a different language, the syntax is mostly compatible but edge cases differ. Python uses a different regex module with slightly different syntax for named groups and Unicode handling.

Conclusion

The Regular Expression Tester is an essential tool for anyone who writes regex as part of their development work. Real-time match highlighting, group capture display, and replace preview together cut down the iteration time dramatically compared to running test code after each change. The MDN regex guide is the reference to keep open alongside the tester when you are working through a complex pattern.

Last reviewed: May 31, 2026

Frequently Asked Questions

What flags should I use in my regex?
The most common flag is g (global), which finds all matches rather than just the first one. Use i for case-insensitive matching. Use m (multiline) to make ^ and $ match the start and end of each line rather than just the entire string. Use s (dotAll) if your pattern uses . and needs it to match newline characters. Use u if you are working with Unicode text that includes characters outside the Basic Multilingual Plane, such as emoji. You can combine multiple flags.
How do capture groups work?
Parentheses in a regex pattern define a capture group. Everything the group matches is captured and available as a separate result alongside the full match. For example, the pattern (\d{4})-(\d{2})-(\d{2}) applied to 2024-06-15 captures the full date as the match, 2024 as group 1, 06 as group 2, and 15 as group 3. Named groups use the syntax (?<year>\d{4}) to give a capture group a readable name you can reference by name rather than index.
Why does my regex match in the tester but not in my code?
The most common cause is a missing or different flag. The tester shows you matches with all selected flags applied; make sure the flags in your code match. Another common issue is escaping: in a JavaScript string literal, backslashes need to be doubled (\d instead of d) unless you are using a regex literal with slashes (e.g. /\d/). Template literals and regular strings both require the double backslash.
What is catastrophic backtracking and how do I avoid it?
Catastrophic backtracking happens when a regex with nested quantifiers or overlapping alternatives tries many possible ways to match a string before concluding it does not match. On a carefully crafted input, this can cause the engine to spend an exponentially long time on a single match attempt. To avoid it, be specific about what each part of your pattern can match, avoid nested quantifiers like (a+)+, and use possessive quantifiers or atomic groups if your regex flavour supports them. Test your patterns against strings designed to cause backtracking before deploying them against user input.
Does this tester work for Python or PHP regex?
The tester runs the JavaScript regex engine, which follows the ECMA-262 standard. JavaScript regex is broadly compatible with PCRE (used in PHP and Perl) and Python's re module, but there are differences in syntax for named groups, lookbehind support, and some Unicode properties. Use this tester for JavaScript code specifically. For other languages, online tools targeting those engines will give more accurate results.
How do I match a literal special character like a dot or plus sign?
Special characters in regex (. * + ? ^ $ { } [ ] | ( ) \) need to be escaped with a backslash to be treated as literals. For example, to match a literal dot, use \. instead of . (which matches any character). To match a literal plus sign, use \+. When writing this in a JavaScript string rather than a regex literal, remember to double the backslash: "\\."

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💡 Pro Tip

Use (?:...) non-capturing groups when you need grouping without capturing the match. This keeps your match results clean and improves performance.

About the Author

S. Siddiqui

S. Siddiqui

Founder & Editor-in-Chief

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S. Siddiqui is the founder and editor-in-chief of YourToolsBase, overseeing all content, tool accuracy, and editorial standards.

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