About Regex Tester & Debugger
BestOfTool's Regex Tester gives you a live workspace to write, test, and debug JavaScript regular expressions directly in your browser — no setup, no data sent to a server.
Regex Tester & Debugger Example
Input
Test text: john.doe@example.com, alice@company.org
Pattern: \b[\w.+-]+@[\w-]+\.[a-zA-Z]{2,}\bOutput
Matches: john.doe@example.com, alice@company.orgHow Regex Tester & Debugger Works
- 1Enter your regular expression pattern in the pattern field — no need to wrap it in forward-slash delimiters.
- 2Select the flags you need (global, case-insensitive, multiline, dotAll, Unicode, sticky) using the flag checkboxes.
- 3Paste or type your test text in the large text area. Matches highlight instantly as you type.
- 4Inspect the match detail cards below to see full matches, positions, and capture group values.
- 5Switch to Replace mode, enter a replacement string, and instantly preview the result using standard JavaScript replacement semantics.
Regex Tester & Debugger Use Cases
- ✓Validate and debug regular expressions for form validation, API data extraction, and log parsing.
- ✓Understand capture group and named group behaviour before using them in production code.
- ✓Test replacement patterns with $1, $2, and $<name> references before committing to code.
- ✓Learn and experiment with JavaScript RegExp flags without writing a test script.
- ✓Quickly verify that a pattern handles edge cases like empty strings, Unicode characters, and multiline text.
Regex Tester & Debugger — Frequently Asked Questions
What is a regular expression?
A regular expression (regex) is a sequence of characters that defines a search pattern. It is used to match, locate, and manipulate text. In JavaScript, regex patterns are created with the RegExp constructor or a literal like /pattern/flags.
Which regex flavor does BestOfTool use?
This tool uses the JavaScript RegExp engine built into your browser. JavaScript regex syntax is similar to PCRE in many common cases, but there are important differences — particularly around lookbehind support, Unicode property escapes, and possessive quantifiers. Patterns tested here use standard JavaScript RegExp semantics. Support for newer regex features can vary by browser or Node.js version.
Does this tool support JavaScript regex?
Yes. The tool exclusively uses the native JavaScript RegExp engine. All evaluation happens in your browser using the same engine that runs your JavaScript code.
What does the g flag do?
The g (global) flag causes the regex engine to find all matches in the test string rather than stopping at the first one. Without g, only the first match is returned. In Replace mode, g determines whether all matches or only the first match are replaced.
What is the difference between the g and i flags?
The g flag controls how many matches are returned (all vs first). The i flag controls case sensitivity — with i, the pattern "hello" matches "Hello", "HELLO", and "hello". The flags are independent and can be combined.
How do capture groups work?
Parentheses in a regex create a capture group. For example, (\w+)@(\w+) has two groups. Group 1 captures the username, Group 2 captures the domain. Captured values are available as $1, $2 in replacements, or as match[1], match[2] in JavaScript code.
Are named capture groups supported?
Yes. Named capture groups use the syntax (?<name>pattern). For example, (?<user>\w+)@(?<domain>\w+) creates groups named "user" and "domain". Named groups can be referenced as $<user> in replacement strings.
How do I replace regex matches?
Switch to Replace mode using the tab at the top of the tool. Enter your replacement string in the Replacement field. Standard JavaScript replacement references work: $& (full match), $1/$2/... (numbered groups), $<name> (named groups). The result updates live as you type.
Does BestOfTool upload my test text?
No. All regex evaluation, highlighting, and replacement happen entirely in your browser using the native JavaScript RegExp engine. Your pattern, test text, and replacement strings are never sent to any server.
Why is my regex invalid?
Common causes of invalid regex errors include: unmatched parentheses like ([a-z, unescaped special characters inside character classes, invalid flag combinations, or incorrect Unicode escape sequences. The error message from the JavaScript engine is shown directly below the pattern field.
Why does my regex match nothing?
If you expect matches but see zero results, check that: (1) the g flag is enabled if you expect multiple matches, (2) the i flag is off if your pattern has explicit case that does not match your text, (3) anchors like ^ and $ behave differently with and without the m flag, and (4) the . character does not match newlines unless the s (dotAll) flag is set.
Does this regex work in Python, Java, or PHP?
Not necessarily. Different programming languages use different regex engines (Python uses re, Java uses java.util.regex, PHP uses PCRE). While many common patterns are portable, features like named group syntax, lookbehind support, possessive quantifiers, and certain Unicode handling differ between engines. Always test your regex in the target environment.
Regex Tester & Debugger Key Features
Live Match Highlighting
Every match is highlighted in real-time as you type your pattern or test text.
Capture Group Inspector
Named and numbered capture groups are displayed individually for each match.
Replace Mode
Preview replacement output with $&, $1, and $<name> references before writing any code.
One-click Examples
Eight common developer patterns — email, URL, UUID, IP, hex colour, ISO date and more.
Flag Controls
Toggle g, i, m, s, u, and y flags with labelled checkboxes and instant feedback.
Privacy First
All evaluation runs in your browser — your pattern and test text never leave your device.
Why Use Our Regex Tester & Debugger?
Regular expressions are one of the most powerful tools a developer can have, but also one of the most error-prone to write by hand. Our Regex Tester removes the guesswork by showing you exactly what matches, where, and what each capture group contains — all updating live as you type.
Whether you are writing a form validation pattern, parsing log files, extracting data from API responses, or learning regex syntax for the first time, the interactive workspace helps you understand exactly what your pattern does before you commit it to code. The included cheat sheet and FAQ make this a useful reference to come back to repeatedly.