Regex Tester:
Real-time pattern matching
Write, test, and debug regular expressions instantly. See matches, capture groups, and replacement previews in real time. All processing runs locally in your browser.
Regex Tester
How it works
Enter your pattern
Type a regular expression into the pattern field and select the flags you need: g for global, i for ignore case, m for multiline, and s for dotAll.
Paste test text
Add the string you want to search in the test area. Matches appear instantly in the matches box, including capture groups when global mode is off.
Test replacements
Optionally enter replacement text to preview the transformed output in real time. Copy the result or refine your pattern until it is perfect.
When developers reach for this tool
From validating form inputs to parsing log files, here are the four situations where the Best Answer Hub Regex Tester saves the most time.
Validating form inputs
Test before production code
Build and test email, phone, postcode, and URL patterns before wiring them into React or Vue form validation. Confirm edge cases (Gmail aliases with + signs, international phone prefixes) without touching production code.
Parsing log files
Extract IPs, timestamps & codes
Extract timestamps, IP addresses, error codes, and status lines from Nginx access logs, AWS CloudWatch output, or application stack traces. Paste a sample log line and iterate on the pattern until capture groups isolate exactly what you need.
Find-and-replace in VS Code
Test before multi-file replace
VS Code's regex search uses the same ECMAScript engine. Test the pattern and replacement string here first, including $1/$2 group references, before running a multi-file replace across an entire codebase.
Security auditing regex patterns
Catch ReDoS vulnerabilities
Check whether patterns in your codebase contain nested quantifiers that could enable a ReDoS attack. The tool warns on dangerous constructs like (a+)+, helping you catch vulnerabilities before Snyk or Dependabot does.
Frequently asked questions
What is the Best Answer Hub Regex Tester?
The Best Answer Hub Regex Tester is a free browser-based utility that lets you write, test, and debug regular expressions in real time. It highlights matches, shows capture groups, and tests replacements instantly using the native JavaScript RegExp engine. Unlike desktop apps, it requires zero installation and works on macOS, Windows, Linux, iOS, and Android. Everything runs locally in your browser. No data is uploaded.
Is the Regex Tester free and safe?
Yes, the Best Answer Hub Regex Tester is completely free with no usage limits, no watermarks, and no signup required. It is safe because all pattern matching happens locally inside your browser using native JavaScript. Your regex patterns and test strings are never sent to a server, stored in a database, or logged anywhere. Disconnect from the internet after loading the page and the Best Answer Hub Regex Tester will continue to work perfectly.
What is a regular expression (regex)?
A regular expression is a sequence of characters that defines a search pattern. Developers use regex to validate email addresses, extract phone numbers from text, parse log files, replace text in Visual Studio Code, and filter data in programming languages like Python, JavaScript, Java, and PHP. Regex is supported by virtually every modern text editor, database engine, and command-line tool including grep, awk, and sed. Paste any pattern into the Best Answer Hub Regex Tester to verify it immediately.
What regex flags does this tool support?
The Best Answer Hub Regex Tester supports four JavaScript regex flags. The g (global) flag finds all matches instead of stopping after the first. The i (ignore case) flag makes matching case-insensitive. The m (multiline) flag makes ^ and $ match the start and end of each line, not just the whole string. The s (dotAll) flag lets the dot (.) match newline characters. The i flag is enabled by default for convenience.
How do capture groups work?
Capture groups let you isolate and extract parts of a match by wrapping them in parentheses. For example, the pattern (d{3})-(d{4}) captures an area code and local number separately. When global mode is off, the Best Answer Hub Regex Tester displays the full match plus each captured group labeled underneath. In JavaScript, you can reference captured groups in replacement strings using $1, $2, and so on.
Global vs non-global regex behavior
With the global (g) flag enabled, the Best Answer Hub Regex Tester lists every match as a separate numbered item. With global disabled, it shows only the first match but reveals all capture groups in that match. This distinction matters in code too: String.match with the g flag returns an array of strings, while match without g returns a Match object with group details. Choose global for counting or listing, and non-global for group extraction.
Can I test replacements?
Yes. Enter a replacement string in the Replace box and the Best Answer Hub Regex Tester instantly previews the result. You can use $1, $2, and so on to insert captured groups, or $& to insert the entire match. For example, replacing (w+) with [$1] wraps every matched word in brackets. This mirrors JavaScript's String.prototype.replace behavior exactly, so what you see here works identically in Node.js or Chrome.
Why is my pattern showing an error?
Common errors include unescaped special characters like +, ?, *, and ., which must be prefixed with a backslash when used literally. Unclosed brackets, parentheses, or curly braces also cause failures. Another frequent mistake is using PCRE-only features like Q...E or recursive patterns, which JavaScript does not support. The Best Answer Hub Regex Tester displays a clear red error message with the exact problem so you can fix it quickly.
Can I use it offline?
Yes. After you load the page once, the Best Answer Hub Regex Tester works without an internet connection. It is built entirely with vanilla JavaScript and uses only browser-native APIs: no external library downloads, no CDN dependencies, and no server-side processing. This makes it ideal for working on sensitive data in restricted or air-gapped environments where you cannot access online services like Regex101.
Common regex patterns for developers (email, phone, URL, IP)
Email validation often uses patterns like [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}. Phone numbers vary by country but US formats often match d{3}-d{3}-d{4}. URL validation might use https?://[w-.]+.w{2,}. IPv4 addresses match (?:d{1,3}.){3}d{1,3}. Paste any of these into the Best Answer Hub Regex Tester to verify or adjust them. For production-grade validation, pair regex with libraries like validator.js or Zod.
How does this compare to Regex101 or regexr.com?
Regex101 and regexr.com are excellent tools with advanced features like explanation panels, regex libraries, and community sharing. The Best Answer Hub Regex Tester focuses on speed, privacy, and simplicity: no ads, no account requirements, and no data leaves your device. It is perfect for quick tests on a colleague's machine, restricted networks, or when you need instant results without navigating a complex interface.
Regex engine compatibility (PCRE, JavaScript, Python)
The Best Answer Hub Regex Tester uses the JavaScript RegExp engine built into your browser, which is ECMAScript compliant. It supports lookaheads and lookbehinds but does not support PCRE features like atomic grouping, possessive quantifiers, or Q...E literal quoting. Python's re module is closer to PCRE and supports named groups with (?P<name>...), while JavaScript uses (?<name>...). If you need PCRE, test your pattern in PHP or Perl before deploying.
Is my test data sent to a server?
No. Your regex patterns and test strings never leave your browser when using the Best Answer Hub Regex Tester. The tool uses only client-side JavaScript to evaluate matches: there are no network requests, no analytics pings containing your data, and no cloud processing. You can verify this by opening the Network tab in your browser's Developer Tools and confirming zero outgoing requests when you type, match, or replace text.
Can I save or share my regex patterns?
The Best Answer Hub Regex Tester does not currently offer built-in saving or sharing. You can bookmark the page and your browser will remember any text left in the fields during the session. For sharing, copy the pattern and flags to a code snippet, Gist, or documentation page. URL-based sharing may be added in a future update. In the meantime, explore the Best Answer Hub Developer Toolbox for more utilities.
What is ReDoS and how can it affect a Node.js application?
ReDoS (Regular Expression Denial of Service) exploits catastrophic backtracking in regex engines. When a vulnerable pattern (typically one with nested quantifiers like (a+)+) is matched against crafted input, backtracking steps grow exponentially, stalling the Node.js event loop for seconds or indefinitely. Real-world CVEs have hit widely-used npm packages including minimatch (CVE-2026-26996), path-to-regexp, picomatch (CVE-2026-33671), and brace-expansion (CVE-2025-5889). OWASP lists ReDoS as a well-known input-validation risk. Mitigations include the safe-regex npm package, vuln-regex-detector, or switching to Google's RE2 engine (which guarantees linear-time matching by rejecting backreferences) via the re2 npm binding. The Best Answer Hub Regex Tester warns when it detects dangerous nested quantifier patterns.
Built & maintained by Shahbaz Ali Malik Last updated: