Regex Tester Tool - Online Regular Expression Tester and Explainer
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Last Updated:
This tool allows you to test regular expressions in real time with instant match highlighting, capture group inspection, and automatic natural language explanation of your pattern. Ideal for debugging, learning, and building regex patterns for JavaScript.
All processing is performed entirely in your browser using client-side JavaScript. No data is transmitted to any server. Your regular expressions and test strings never leave your device.
- This tool is provided "AS IS" without any warranties of any kind.
- The author accepts no responsibility for incorrect matches or pattern explanations.
- This tool uses JavaScript's built-in RegExp engine; behavior may differ from other regex flavors (PCRE, Python, etc.).
- Always verify critical regex patterns with your target environment's regex engine.
- By using this tool, you accept full responsibility for any outcomes.
This tool uses client-side JavaScript for all processing. No data is transmitted to servers, no files are uploaded online, all processing happens locally in your browser. Once loaded, this tool continues to work even without an internet connection. For more details, please refer to our Web Tools Disclaimer.
Match Highlights
Match Details
Pattern Explanation
▶ Show Quick Reference
| Syntax | Description | Example |
|---|---|---|
. | Any character (except newline) | a.c matches "abc", "a1c" |
\d | Digit (0-9) | \d{3} matches "123" |
\w | Word character (a-z, A-Z, 0-9, _) | \w+ matches "hello" |
\s | Whitespace (space, tab, newline) | \s+ matches " " |
\b | Word boundary | \bword\b matches "word" |
^ | Start of string/line | ^Hello matches "Hello world" |
$ | End of string/line | world$ matches "Hello world" |
[abc] | Character class | [aeiou] matches vowels |
[^abc] | Negated character class | [^0-9] matches non-digits |
* | Zero or more | ab*c matches "ac", "abc", "abbc" |
+ | One or more | ab+c matches "abc", "abbc" |
? | Zero or one (optional) | colou?r matches "color", "colour" |
{n} | Exactly n times | \d{4} matches "2026" |
{n,m} | Between n and m times | \d{2,4} matches "12", "123", "1234" |
(...) | Capturing group | (\d+)-(\d+) captures "123" and "456" |
(?:...) | Non-capturing group | (?:ab)+ matches "abab" |
a|b | Alternation (or) | cat|dog matches "cat" or "dog" |
(?=...) | Positive lookahead | \d(?=px) matches "3" in "3px" |
(?!...) | Negative lookahead | \d(?!px) matches "3" in "3em" |
(?<=...) | Positive lookbehind | (?<=\$)\d+ matches "100" in "$100" |
(?<name>...) | Named capturing group | (?<year>\d{4}) captures as "year" |
Features
- Real-time regex matching with instant visual highlighting of all matches
- Support for all JavaScript regex flags: global (g), case-insensitive (i), multiline (m), dotAll (s), unicode (u), and hasIndices (d)
- Detailed match information including match value, position index, and length
- Capture group display for both numbered and named groups
- Automatic natural language explanation of your regex pattern using a built-in recursive descent parser
- Quick reference guide for common regex syntax
- Debounced input for smooth real-time updates
- Color-coded match highlights with alternating colors for easy identification
- Complete client-side processing with no server communication
How to Use
- Enter your regular expression pattern in the "Pattern" field (without the surrounding slashes).
- Select the desired flags using the checkboxes (global flag is enabled by default).
- Type or paste your test string in the "Test String" area.
- Matches are highlighted automatically in real time as you type.
- Review the "Match Details" section for detailed information about each match, including capture groups.
- Read the "Pattern Explanation" section for a natural language breakdown of what your regex does.
- Use "Copy Match Details" to copy the match results to your clipboard.
- Click "Show Quick Reference" for a handy guide to common regex syntax.
Important Notes
- This tool uses JavaScript's built-in RegExp engine. Regex behavior may differ from other flavors such as PCRE (PHP/Perl), Python, Java, or .NET.
- Features like atomic groups, possessive quantifiers, and conditional patterns are not supported in JavaScript regex.
- The
d(hasIndices) flag may not be supported in older browsers. - Very complex patterns or extremely long test strings may cause the browser to become temporarily unresponsive.
- The pattern explanation is a best-effort analysis and may not cover every edge case of regex syntax.
References:
Tech Blog with curated related content
Web Tools Collection
Written by Hidekazu Konishi