🔍 Regex Tester
Test regular expressions with real-time highlighting and capture groups.
Quick Reference
.Any character (except newline)
\dDigit [0-9]
\wWord char [A-Za-z0-9_]
\sWhitespace
^Start of string/line
$End of string/line
*0 or more
+1 or more
?0 or 1
{n,m}Between n and m times
[abc]Character class
(abc)Capture group
(?:abc)Non-capturing group
a|bAlternation (a or b)
(?=abc)Positive lookahead
(?!abc)Negative lookahead
Free Regex Tester
Test and debug regular expressions in real-time. See matches highlighted, view capture groups, and use our built-in quick reference. Supports all JavaScript regex flags including global, case-insensitive, multiline, dotAll, and unicode. All processing happens in your browser -your data never leaves your device.
What is a Regex Tester?
A regex tester is a tool that lets you write, test, and debug regular expressions against sample text in real time. Regular expressions (regex or regexp) are patterns used to match, search, and manipulate strings in virtually every programming language. They are essential for tasks like form validation, log parsing, data extraction, and text processing. However, regex syntax can be complex, and even small mistakes can produce unexpected matches or miss valid patterns entirely.
This online regex tester provides instant visual feedback as you type your pattern. Matches are highlighted directly in your test string, capture groups are displayed in detail, and errors in your pattern syntax are caught immediately. The tool supports all standard JavaScript regex flags and runs entirely in your browser, so your test data is never sent to any external server. It is a valuable companion for both beginners learning regular expressions and experienced developers fine-tuning complex patterns.
How to Use This Tool
- Type your regular expression pattern in the input field between the forward slashes. The tool uses JavaScript regex syntax, so patterns like \d+, [A-Za-z]+, and (?:group) all work as expected.
- Set your desired flags using the toggle buttons or by typing directly in the flags field. Common options include g (global), i (case-insensitive), m (multiline), s (dotAll), and u (unicode).
- Paste or type your test string in the text area below. Matches are highlighted instantly with alternating colors so you can distinguish overlapping or adjacent matches at a glance.
- Review the match details section to see each match's value, position index, and any captured groups. The quick reference panel at the bottom provides a handy cheat sheet for common regex syntax.
Key Features
- Real-Time Highlighting - Matches appear highlighted in your test string as you type, with alternating colors to differentiate adjacent matches.
- Capture Group Display - Each match shows its captured groups with numbered references ($1, $2, etc.), making it easy to verify that your groups extract the right data.
- All JavaScript Flags - Supports global (g), multiline (m), case-insensitive (i), dotAll (s), and unicode (u) flags with convenient toggle buttons.
- Instant Error Feedback - Syntax errors in your pattern are caught and displayed immediately, helping you fix issues before copying the regex into your code.
- Built-in Quick Reference - A collapsible cheat sheet covers the most commonly used regex tokens, character classes, quantifiers, anchors, and lookaheads.
Frequently Asked Questions
Which regex engine does this tool use?
This tool uses the built-in JavaScript (ECMAScript) regular expression engine in your browser. While regex syntax is largely similar across languages, there are some differences. For example, JavaScript does not support lookbehind in older browsers, and features like named groups use a slightly different syntax than PCRE (Perl-Compatible Regular Expressions). Always verify your patterns in the target language if you plan to use them outside JavaScript.
What does the global (g) flag do?
The global flag tells the regex engine to find all matches in the test string rather than stopping after the first match. Without the g flag, only the first occurrence is highlighted. For most testing and data extraction tasks, you will want the global flag enabled so you can see every instance of your pattern.
Can I test regex patterns for Python or Java here?
You can test the core pattern logic here since most basic regex features (character classes, quantifiers, groups, alternation) work the same way across languages. However, language-specific features like Python's re.VERBOSE mode or Java's possessive quantifiers are not available in JavaScript. Use this tool for prototyping and debugging your patterns, then validate in your target environment for edge cases.
Is there a limit to how much text I can test?
There is no hard limit on the test string size. However, very large texts combined with complex patterns (especially those with excessive backtracking) may cause the browser to slow down. The tool caps match display at 1,000 matches to prevent performance issues. For most practical use cases, the tool handles thousands of lines of text without any problems.