Regex tester, named groups, replacement preview and saved patterns
Test JavaScript regular expressions against real sample text, inspect matches, indexes, numbered groups and named groups, preview replacements, load common pattern starters, save patterns locally, copy snippets and review safety notes before putting regex into production code.
Regex runs in your browser with the JavaScript engine. Other languages may use slightly different syntax, escaping and feature support.
A regex tester should show what the pattern actually did
Regular expressions are powerful because they are compact, but that compactness can hide mistakes. A pattern may match the first example and still fail on multiline logs, Unicode text, named groups, replacement output or a near miss that should be rejected. A useful regex tester should show match indexes, captured groups, named groups, replacement preview, flags and test cases before you copy anything into production.
This regex tester focuses on that workflow. It runs JavaScript regex locally, highlights matches in sample text, lists match start and end positions, shows numbered and named captures, previews replacement output, loads practical starter patterns, saves patterns in browser storage and generates code snippets for JavaScript, PHP-style PCRE and Python with named-group conversion notes.
How to test a regular expression safely
Start with real sample text, not only a perfect example. Add positive lines that must match and negative lines that must fail. Decide whether the pattern should find values inside text or validate a whole field. Review flags carefully: global matching changes how many results you collect, multiline changes anchors, dotAll changes what a dot can consume, and case-insensitive mode can hide data assumptions.
- Matches show each value, start index and end index.
- Groups show numbered captures and named captures when the engine supports them.
- Replace preview catches destructive cleanup mistakes before they reach a file or database.
- Saved patterns keep local drafts in the browser for repeat testing.
- Safety notes highlight greedy wildcards, nested quantifiers and parsing jobs that need a real parser.
Common regex debugging examples
If a log cleanup removes too much text, look for greedy .* sections and test several lines. If an email pattern is used for account creation, remember that regex is only a first filter and confirmation still matters. If a URL pattern grabs punctuation, adjust the boundary or trim punctuation in code. If you need to parse JSON, HTML or full programming languages, use a parser and reserve regex for smaller text tasks.
Common questions
Are JavaScript regex and PCRE the same?
No. They overlap, but flags, lookbehind, named groups and escaping can differ. Always test in the target runtime.
Can regex validate every email address?
Not perfectly in a practical way. Use regex as a first filter, then confirm ownership or use application-specific validation.
Can regex parse HTML or JSON?
Regex can find small fragments, but full parsing should use a proper parser.
Which regex flavor does this tester use?
JavaScript (ECMAScript) regular expressions, because it runs in your browser. Some PCRE or Python features such as certain lookbehind or named-group syntaxes may behave differently.
What do the g, i and m flags do?
g matches all occurrences instead of the first, i makes matching case-insensitive, and m makes the anchors ^ and $ match at the start and end of each line.
Why is my regex slow or freezing the page?
Catastrophic backtracking, usually from nested quantifiers like (a+)+ on non-matching input. Rewrite the pattern to remove ambiguous overlap.
