Regex Tester

Test JavaScript regular expressions against text with flags, capture groups, named groups, and match highlighting.

For this tool, processing runs locally in your browser and data is not sent to the server.

Regex Tester is a free online tool for testing JavaScript regular expressions against text. It supports all RegExp flags, shows matches with start/end positions, capture groups, and named groups, and highlights results in the text. The tool runs in your browser and does not send data to the server.

What it does

Output

Processing

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How to use

Enter a regular expression without outer slashes, or paste it in /pattern/flags format.
Select flags: global, ignore case, multiline, dotAll, unicode, or sticky.
Paste the test text and click Test.
Review the match count, highlighting, positions, capture groups, and named groups.

More about this tool

Regex Tester helps you test a regular expression against text directly in the browser. The tool supports JavaScript RegExp, the g, i, m, s, u, and y flags, and shows all matches with their start/end positions, capture groups, and named groups. It is useful for debugging filters, form validation, searching for values in logs, validating emails, URLs, dates, IDs, and text patterns.

To protect the browser, the tool applies limits on text length and the number of matches, guarding against catastrophic backtracking from complex expressions.

All processing is done locally in the browser. Your data is not sent to the server.

Runs in the browser
Data is not sent to the server
Tests JavaScript regex
Supports g, i, m, s, u, y flags
Shows matches and positions
Shows capture groups and named groups
Highlights matches in the text
Uses limits to prevent browser hanging
No registration required

Frequently Asked Questions

How do I test a regular expression?

Enter the pattern in the regex field, select flags, paste the test text, and click Test. The tool shows matches, positions, and groups.

Do I need to include outer slashes in my regex?

You can enter just the pattern without outer slashes, for example \d+. If you paste /\d+/gi format, the tool can automatically extract the pattern and flags.

What does the g flag do?

The g flag enables finding all matches. Without it, JavaScript RegExp typically returns only the first match.

What does the i flag do?

The i flag enables case-insensitive matching, so Text, text, and TEXT are all treated as matches.

What does the m flag do?

The m flag changes ^ and $ so they match the start and end of each line, not just the entire string.

Does the tool show capture groups?

Yes. The tool shows both numbered capture groups and named groups if they are present in the regex.

Why can a regex hang on large text?

Some expressions with nested quantifiers can cause very long execution (catastrophic backtracking). The tool applies text length and match count limits to prevent this.

Is this PCRE or JavaScript regex?

The tool uses JavaScript RegExp in the browser. The syntax may differ from PCRE, Python, PHP, .NET, and other regex engines.

Can I replace text using a regex?

This tool is for testing regular expressions. For text replacement, use a dedicated Find and Replace or Regex Replace tool.

Is my data sent to the server?

No. The regex and test text are processed locally in the browser and are not sent to the server.

Do I need to register?

No. The tool works immediately without registration.

More about this tool

To protect the browser, the tool applies limits on text length and the number of matches, guarding against catastrophic backtracking from complex expressions.

All processing is done locally in the browser. Your data is not sent to the server.

Runs in the browser
Data is not sent to the server
Tests JavaScript regex
Supports g, i, m, s, u, y flags
Shows matches and positions
Shows capture groups and named groups
Highlights matches in the text
Uses limits to prevent browser hanging
No registration required

Frequently Asked Questions

How do I test a regular expression?

Enter the pattern in the regex field, select flags, paste the test text, and click Test. The tool shows matches, positions, and groups.

Do I need to include outer slashes in my regex?

You can enter just the pattern without outer slashes, for example \d+. If you paste /\d+/gi format, the tool can automatically extract the pattern and flags.

What does the g flag do?

The g flag enables finding all matches. Without it, JavaScript RegExp typically returns only the first match.

What does the i flag do?

The i flag enables case-insensitive matching, so Text, text, and TEXT are all treated as matches.

What does the m flag do?

The m flag changes ^ and $ so they match the start and end of each line, not just the entire string.

Does the tool show capture groups?

Yes. The tool shows both numbered capture groups and named groups if they are present in the regex.

Why can a regex hang on large text?

Some expressions with nested quantifiers can cause very long execution (catastrophic backtracking). The tool applies text length and match count limits to prevent this.

Is this PCRE or JavaScript regex?

The tool uses JavaScript RegExp in the browser. The syntax may differ from PCRE, Python, PHP, .NET, and other regex engines.

Can I replace text using a regex?

This tool is for testing regular expressions. For text replacement, use a dedicated Find and Replace or Regex Replace tool.

Is my data sent to the server?

No. The regex and test text are processed locally in the browser and are not sent to the server.

Do I need to register?

No. The tool works immediately without registration.