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Regex Tester Online with Live Match Highlighting

Build, debug and save regular expressions with timeout-protected live matching, exact positions, capture groups and replacement previews.

Regular-expression testing workspace

Live highlights, safe execution, replacement testing and persistent sessions

Ready. The included sample demonstrates named groups, global matching and replacement output.
Timeout protection is active.
gim
Enter the pattern body without slash delimiters. Maximum 10,000 characters.
Flags

Test text

0 characters · 0 linesTyped text

Live match highlight

Run a pattern to highlight matches.
No result yet0 highlighted
0Matches
0Unique values
0Captured groups
0 msExecution time
—First index

Match details

No matches to display.

Replacement preview

$& whole match
$1 numbered group
$<name> named group
$$ literal dollar
Result
Run the test to preview replacement output.

Replacement uses the selected flags. Without the global flag, only the first applicable match is replaced; sticky mode must also begin at the selected start index.

Name session

Use a clear name such as Email validation or Log parser.

Regex tester with safer live execution

Use the tester to inspect what a regular expression actually matches, where each match starts and ends, which capture groups are populated, and how a replacement template changes the text.

Live visual feedback

Matched ranges are highlighted beside the source text, while zero-length matches receive a visible position marker.

Detailed match records

Each result includes its index, end position, line, column, value, numbered groups and available named groups.

Timeout protection

Tests run away from the interface and are stopped when they exceed the chosen execution limit, reducing the risk of a frozen page.

Replacement testing

Preview standard replacement tokens before moving a pattern into application code, an editor or a server workflow.

Sticky named sessions

Keep multiple patterns and test documents organized locally, then rename, duplicate, export, import or delete them.

Developer-friendly exports

Copy a regex literal or download match reports, replacement output and validated session data with prefixed filenames.

How to use the regex tester

Follow the same workflow shown by the controls in the testing workspace.

1

Enter a pattern and flags

Type the regular-expression pattern without slash delimiters, then select the flags required by the test.

2

Add test text

Paste text, load a preset, or import a supported UTF-8 text file into the test-text editor.

3

Review live matches

Run the test or keep live testing enabled to inspect highlighted matches, positions, capture groups and performance details.

4

Test a replacement

Enter a replacement template and review the resulting text before copying or downloading it.

5

Save or export the session

Keep named sessions on the current device, export a session file, or download match results for later review.

Regular-expression syntax quick reference

Common building blocks for browser-based regular expressions. Exact advanced-feature support depends on the current browser.

PatternMeaningExample use
.Any character except a line terminator unless dot-all mode is enableda.c
\d / \DDigit / non-digit\d{4}
\w / \WWord character / non-word character\w+
\s / \SWhitespace / non-whitespace\s+
[abc]One character from a set[A-F0-9]
[^abc]One character not in a set[^,\r\n]+
^ / $Start / end boundary, affected by multiline mode^\w+$
* + ?Zero or more, one or more, zero or onehttps?
{m,n}Minimum and maximum repetition\d{2,4}
(...)Numbered capture group(\w+)@
(?<name>...)Named capture group(?<year>\d{4})
(?:...)Non-capturing group(?:cat|dog)
x|yAlternativeyes|no
(?=...) / (?!...)Positive / negative lookahead\w+(?=:)
\bWord boundary\bword\b

Performance, privacy and practical limitations

Performance boundaries

A timeout stops a long-running browser test, and the match limit controls result volume. Neither feature proves that the expression will be efficient in production or on much larger input.

Flavor differences

The tester follows current browser syntax. Patterns copied from another language may require different escaping, flags, anchors, Unicode handling or replacement tokens.

Local workspace processing

The page has no tool-specific regex-processing endpoint. Test text, replacement content and session exports are produced in the current browser.

Optional persistent sessions

Remember sessions stores pattern and text data for this website on the current device. Disable it or clear saved data before using a shared computer.

Compatibility and troubleshooting

Invalid pattern

Review the highlighted error message, unmatched parentheses, character classes, backslashes and unsupported constructs.

Timed-out test

Reduce the text size, lower ambiguous nesting, replace broad repeated groups, or test smaller portions of the expression.

No matches

Check case sensitivity, anchors, line mode, start index, escaping and whether sticky mode requires a match at the exact current position.

Session did not persist

Confirm Remember sessions is enabled and that browser storage is available. Private browsing may clear data after the private session ends.

Clipboard unavailable

Use the download actions when clipboard access is unavailable. Copy operations generally work best on a secure HTTPS page.

Imported text rejected

Save it as UTF-8 text, remove binary content, and keep the file below 2 MB and 300,000 decoded characters.

Regex tester frequently asked questions

Answers about flags, highlighting, capture groups, replacement strings, timeout protection, sessions, imports and privacy.

What is this regex tester?

It is a browser-based regular-expression workspace with live highlighting, match details, capture groups, replacement preview, timeout protection and named sessions.

Does it test the pattern while I type?

Yes. Live testing is enabled by default and runs after a short typing delay. You can pause live testing and use the Run Test button manually.

Should I include slash delimiters around the pattern?

No. Enter only the pattern body. Select flags separately using the flag controls beside the pattern.

Which flags are available?

The page exposes the flags supported by the current browser, including global, case-insensitive, multiline, dot-all, Unicode, Unicode sets, sticky and match-index modes where available.

Why is a flag disabled?

A disabled flag is not supported by the current browser, or it conflicts with another selected mode. Unicode and Unicode-sets modes cannot be active together.

What does the global flag do?

Global mode continues searching after the first match so the tester can collect every non-overlapping match up to the selected result limit.

What does the sticky flag do?

Sticky mode requires a match to begin exactly at the current search position. It is useful for tokenizers and parsers that advance through text in a controlled sequence.

What is the match-indices flag?

When supported, match-indices mode exposes exact start and end positions for the whole match and capture groups. The tester may also use it internally for accurate highlighting.

Does the tester support named capture groups?

Yes, when the current browser supports them. Named groups appear beside numbered capture groups in the match details.

Can I test lookahead and lookbehind?

Lookahead is broadly supported. Lookbehind depends on the current browser, so a pattern may be rejected when that syntax is unavailable.

Does it support PCRE-only syntax?

No. The tester follows the regular-expression syntax implemented by current web browsers. Some PCRE, Python, Java and .NET constructs have different syntax or are unavailable.

How are zero-length matches displayed?

A zero-length match is shown as a small position marker because there is no character range to color. Its exact index remains visible in the match list.

Can the tester find overlapping matches?

Ordinary matching returns non-overlapping results according to browser regular-expression behavior. Overlapping matches usually require a lookahead-based pattern.

What prevents a slow pattern from freezing the page?

Pattern execution runs away from the main interface and is stopped when it exceeds the selected timeout. A new test also cancels the previous run.

Does timeout protection prove that a regex is safe?

No. It limits one browser test, but production performance depends on input size, runtime, pattern structure and surrounding application logic.

What is catastrophic backtracking?

It is excessive repeated searching caused by certain ambiguous nested patterns. The page warns when a test times out, but developers should still simplify risky expressions.

How many matches can be displayed?

You can choose a practical limit from 100 to 10,000 matches. The tester stops collecting at that limit to protect responsiveness.

What is the maximum test-text size?

The workspace accepts up to 300,000 characters. Imported files must be 2 MB or smaller and decode as UTF-8 text.

Which files can I import?

You can import UTF-8 TXT, LOG, JSON, CSV, Markdown, XML, HTML and source-code text files. Binary content is rejected.

Can I test replacement strings?

Yes. Enter a replacement template and the page produces a live replacement result using the selected pattern and flags.

Are replacement tokens supported?

Yes. Standard replacement tokens such as the whole match, numbered captures and named captures work when supported by the current browser.

Can I copy the regex as a literal?

Yes. Copy Regex copies a slash-delimited representation with escaped slash characters and the currently selected flags.

Can I download match results?

Yes. The page can download a prefixed TXT report or CSV file containing match numbers, positions, values and capture-group information.

What are sticky sessions?

Named sessions remember the pattern, flags, test text, replacement and settings on this device so you can return to them after closing the page.

Where are saved sessions stored?

When Remember sessions is enabled, sessions are saved in browser storage for this website. They are not sent to a tool-specific server endpoint.

Can I disable session storage?

Yes. Turn off Remember sessions on this device. The page removes its stored session data while keeping the current in-memory workspace available until the tab closes.

Can I export or import a regex session?

Yes. Export Session creates a JSON file, and Import Session restores a compatible session file after validation.

Does deleting a session remove its saved text?

Yes. After confirmation, the named session is removed from the in-memory list and from browser storage when session saving is enabled.

Is my test text uploaded?

This page has no tool-specific upload or regex-processing endpoint. Typed and imported content is processed in the current browser.

Does the page contact external services?

The regex workspace does not require an external processing service or page-specific dependency. Shared website resources may still load normally.

Which browsers are expected to work?

Current stable Chrome, Edge, Firefox and Safari support the core tester, local sessions, text import, copy and download workflows. Individual regex features depend on browser support.

Why does a valid pattern behave differently elsewhere?

Regular-expression flavors differ. Flags, Unicode handling, anchors, replacement tokens and advanced constructs can vary between browsers, servers, editors and programming languages.

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