What is the difference between a match and a capture group?

A match is the full matched substring, while capture groups defined with parentheses extract specific sub-portions separately.

Regex Tester - Live Match Highlighting

Q: Why do I need to add the g flag to find all matches?

Without the global flag, the regex engine stops after finding the first match; adding g continues searching for every match.

Q: Is my regex pattern or test text sent to a server?

No, everything runs entirely within the browser using its built-in regex engine with nothing transmitted anywhere.

What Is a Regex Tester?

This tool runs your regular expression against real test text using your browser's actual JavaScript regex engine - the exact same engine that runs regex in any JavaScript application. Matches are highlighted directly in your text, and every capture group is listed out explicitly, giving you a genuine, accurate view of exactly what your pattern matches and captures, not an approximation from a simplified regex simulator.

How to Use the Regex Tester

Type your regular expression pattern into the first field.
Add flags like g (global), i (case-insensitive), or m (multiline) in the second field.
Paste or type your test text into the larger box below.
Matches highlight live in the text, with a full list of matches and any capture groups shown underneath.

How It Works

This tool constructs a genuine JavaScript RegExp object from your pattern and flags, then runs it against your test text using the standard exec() method, looping through all matches when the global flag is set. Each match's exact position, matched text, and any capture group values are extracted directly from the real match result object. The highlighted view is built by inserting your actual matched substrings, wrapped in a highlight marker, back into the original text at their exact original positions.

Understanding Your Results

Matches highlight exactly where expected - your pattern is working correctly against the test text as written.
"Invalid regex" error appears - your pattern has a syntax error, such as an unclosed bracket or group; the specific error message describes what the JavaScript regex engine couldn't parse.
No matches found, but you expected some - double-check your flags (particularly whether case-insensitive matching is needed) and confirm special regex characters are properly escaped if you intended them literally.
Capture groups show "undefined" - that specific group didn't participate in the match, which happens naturally with optional groups or alternation patterns where only one branch matched.

Browser Limitations

This tool uses JavaScript's regex engine specifically, which has some syntax differences from other languages' regex implementations, like Python or PCRE - most common patterns work identically, but certain advanced features can differ slightly between engines. Extremely complex patterns with catastrophic backtracking potential could run slowly against very long test strings, a general characteristic of regex engines rather than something specific to this tool.

Common Mistakes to Avoid

Forgetting the global flag when expecting multiple matches - without the g flag, only the first match is found even if the pattern would match multiple times.
Not escaping special regex characters meant to be literal - characters like ., *, and ( have special meaning in regex; escape them with a backslash if you want to match them literally.
Assuming regex syntax is identical across all programming languages - while very similar, JavaScript regex has some specific differences from other languages that can occasionally cause surprises when porting a pattern between them.

Frequently Asked Questions

Why do I need to add the "g" flag to find all matches?

Without the global flag, JavaScript's regex engine stops after finding the first match; adding g tells it to continue searching and find every match in the text.

What's the difference between a match and a capture group?

A match is the full substring your entire pattern matched, while capture groups (defined with parentheses in your pattern) let you extract specific sub-portions of that match separately.

Is my regex pattern or test text sent to a server?

No, everything runs entirely within your browser using its built-in regex engine - nothing you type is transmitted anywhere.

Does JavaScript regex work exactly like Python or other languages' regex?

They're very similar for most common patterns, but some specific syntax and features differ between regex engines across languages, so a pattern tested here should generally work in JavaScript specifically rather than being assumed universal.

Why does my pattern seem to hang on long text?

Certain complex patterns can exhibit slow performance on specific input through a phenomenon called catastrophic backtracking; simplifying the pattern or being more specific with quantifiers usually resolves this.

Conclusion

Testing against a real regex engine with live highlighting takes the guesswork out of writing and debugging patterns. For working with structured text data more broadly, check the JSON Formatter & Validator.