Which regex flavor does this use?

It uses JavaScript's native RegExp engine, which supports ECMAScript regular expressions including lookbehinds, named groups, and Unicode properties.

Can I test replace patterns?

Yes. Enter a replacement string and the tool shows a live preview of the substituted result, supporting backreferences like $1, $2 and named backreferences like $ .

Absolutely. All regex processing happens locally in your browser. No data is transmitted to any server.

Does it support PCRE regex?

This tool uses JavaScript's regex engine, which is based on ECMAScript and differs slightly from PCRE (Perl Compatible Regular Expressions). Most common patterns work identically, but some PCRE-specific features like recursive patterns are not supported.

Regex Tester & Matcher

Evaluate exact JS regular expressions, extract capturing groups, and preview global string replacements natively.

Regular Expression

Test String Input

Length: 55 chars6 Matches

Highlighted Match Execution

Hello World, this is a Regex tester. Match These Words!

Capturing Groups (6)

Match 1Index: 0

Full Match

Hello

Group 1

Match 2Index: 6

Full Match

World

Group 1

Match 3Index: 23

Full Match

Regex

Group 1

Match 4Index: 37

Full Match

Match

Group 1

Match 5Index: 43

Full Match

These

Group 1

Match 6Index: 49

Full Match

Words

Group 1

Evaluate Replacement

Replace with:

, this is a tester. !

Building a validation pattern for email addresses, URLs, or log parsing? Type your regex and test string here — matches highlight instantly as you type, with full capture group extraction and replacement preview.

Last Updated: May 12, 2025Privacy: 100% Local Browser Processing

What is the Regex?

A Regex Tester is a developer tool for writing, testing, and debugging regular expressions interactively. Regular expressions (regex) are patterns used to match character combinations in strings — essential for input validation, text parsing, log analysis, and search-and-replace operations. This tool highlights all matches in your test string in real time, shows captured groups, supports all standard JavaScript regex flags, and provides a live replace preview.

Real-World Use Cases

Input validation — build and test patterns for email addresses, phone numbers, URLs, IP addresses, credit card numbers, and other structured input before implementing them in production code.
Log parsing — develop regex patterns to extract timestamps, error codes, IP addresses, and request paths from server log files (Nginx, Apache, application logs).
Data extraction — build patterns to scrape specific data from unstructured text, HTML, or CSV files.
Search and replace — test complex find-and-replace operations with capture groups before running them on production data.
Code refactoring — develop patterns for IDE-level find-and-replace to rename variables, update function signatures, or migrate API calls across a codebase.

How to Use

1Enter your regular expression pattern in the pattern field.
2Type or paste your test string in the input area.
3Matches are highlighted in real time as you type.
4View captured groups for each match in the results panel.
5Use the Replace field to preview substitution results.
6Toggle flags (global, case-insensitive, multiline) as needed.

Technical Deep Dive

Regular expressions are one of the most powerful — and most misunderstood — tools in a developer's arsenal. They are implemented as finite state machines that process input strings character by character, following transition rules defined by the pattern. This tester uses JavaScript's native RegExp engine, which implements the ECMAScript specification for regular expressions. This includes support for advanced features like lookbehinds (?<=...), named capture groups (?<name>...), Unicode property escapes (\p{Letter}), and the dotAll flag (s) which makes the dot metacharacter match newline characters. The tool compiles your pattern into a RegExp object in real time, executing it against the test string on every keystroke. Each match is extracted along with its capture groups and index positions, then rendered as highlighted overlays on the test string. Invalid patterns are caught via try-catch around the RegExp constructor, and the error message is displayed without crashing the tool. The replace preview applies your replacement string using the String.prototype.replace() method, supporting backreferences ($1, $2) and named backreferences ($<name>).

Pro Tips & Best Practices

Always start with a simple pattern and add complexity incrementally. Test after each change to catch issues early.
Use named capture groups (?<year>\d{4}) instead of numbered groups for readability — they make your regex self-documenting.
Be aware that JavaScript regex does not support possessive quantifiers (++) or atomic groups (?>...) — use lookaheads as alternatives.
When matching across lines, remember to enable the multiline (m) flag to make ^ and $ match line boundaries, not just string boundaries.
If your pattern seems correct but matches nothing, check if you need the global (g) flag for multiple matches or the case-insensitive (i) flag.