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Regex Builder & Tester
Visual Regex Builder & Tester
Build regular expressions visually with blocks and test matches in real-time. 100% client-side operations.
100% Offline
Business
Quick Tokens
Click a token below to append it to your active regex pattern:
Cheat Sheet
.
Any single char
a|b
Either a or b
(...)
Capture Group
(?:...)
Non-capture Group
\b
Word Boundary
(?=...)
Positive Lookahead
Expression Editor
Valid Regex
/
/
Test Text
Real-Time Highlights
Language Snippets
// JS loading...Matches Summary
0
Total Matches
0
Groups/Match
Matches List
No active matches found. Enter valid expression and text inputs.
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Frequently Asked Questions
Quick direct-answer guides about using this utility tool locally and securely.
Does the visual regex builder transmit my test text to any servers?
No, the regular expression execution and match highlighting are executed entirely client-side using your browser's built-in V8 Javascript regular expression engine. Your sample text is never uploaded.
What regular expression standard is used for matching?
The tool uses standard Javascript RegExp specs. Minor variations might exist compared to PCRE (PHP) or Python's `re` engine, but the core token syntax remains identical.
What do the regex flags g, i, and m mean?
g (global) finds all matches instead of stopping at the first one, i (ignore case) makes matching case-insensitive, and m (multiline) makes ^ and $ match at the start/end of each line instead of the whole string.
Why is my regex pattern not matching anything?
The most common causes are an unescaped special character (like ., +, ?, (, or [) that needs a backslash, or forgetting the g flag when you expect multiple matches instead of just the first one.
What is a capture group in regex?
Parentheses () in a pattern create a capture group, letting you extract a specific portion of the match — for example, (\w+)@(\w+)\.com captures the username and domain separately from an email address, accessible as $1 and $2.
Does this tool support lookahead and lookbehind assertions?
Yes, since it uses the JavaScript (ECMAScript) regex engine, both positive/negative lookahead (?=...) (?!...) and lookbehind (?<=...) (?<!...) are supported in all modern browsers.
What are some common regex patterns for validating email, phone, or URLs?
Common starting patterns include ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ for email and ^\d{3}-\d{3}-\d{4}$ for a US phone format — though for real-world validation, pairing a simple regex with server-side confirmation (like an email verification link) is safer than over-restricting the pattern.
Why does my regex work in Python but not in this tester (or vice versa)?
Because different programming languages use different regex engines — this tool uses JavaScript engine, and while most syntax is shared, features like lookbehind, named groups, and Unicode handling can be written slightly differently in PCRE (PHP), Python's re module, or Java.
What does "catastrophic backtracking" mean and how do I avoid it?
It happens when nested quantifiers (like (a+)+b) cause the regex engine to try exponentially many combinations when a match fails, making the pattern extremely slow. Avoid it by being specific ([a-z]+ instead of .+), anchoring your pattern, and using non-capturing groups (?:...) when you don't need the matched text.