How to Use the Regex Tester & Visualizer
Regular expressions (RegEx) are essential string processing patterns used across web development, backend validation, data cleaning, and log analysis. This interactive editor allows developers to test ECMAScript regular expressions in real time, inspect capture groups, and verify text replacements securely inside browser memory.
⚡ 100% In-Browser Privacy
Zero test strings, patterns, or replacement values leave your machine. All calculations run client-side using JavaScript RegExp APIs.
🎯 Visual Match Highlighting
Instantly highlights exact matching ranges, start/end byte offsets, and captures structured sub-expressions in detailed tables.
🔄 Instant Substitution Preview
Preview text replacement and variable insertion (such as $1, $2, or named groups) with real-time reactive feedback.
Step-by-Step Instructions
- Step 1: Write Pattern or Load Preset: Enter your regex pattern and select active flags (g, i, m, s, u), or click one of our predefined presets like Email or URL.
- Step 2: Input Test String: Type or paste sample text into the input textarea to instantly observe visual match highlighting.
- Step 3: Inspect Capture Groups: Review the Match Details table to verify index bounds and sub-group captures ($1, $2).
- Step 4: Test Replacement & Copy: Test string substitution patterns and use the copy buttons to extract matches or transformed outputs into your codebase.
Technical Deep-Dive: ECMAScript Regex Syntax & Flags
JavaScript regular expressions follow ECMAScript standards with rich support for flags and modern character classes:
| Flag | Name | Description |
|---|---|---|
g | Global | Find all matches rather than stopping after the first match. |
i | Case Insensitive | Perform case-insensitive matching across A-Z and a-z. |
m | Multiline | Treat anchor characters ^ and $ as beginning and end of each line. |
s | dotAll | Allows the dot . character to match newline linebreaks. |
u | Unicode | Enables full Unicode property escapes (e.g., \p{Emoji}). |
Frequently Asked Questions
Is my data private and secure?
Yes, 100%. All processing occurs completely in your browser memory using native client-side JavaScript APIs. No text or regex patterns are sent to remote servers.
Which regex engine does this tool use?
This tool uses your browser native ECMAScript JavaScript RegExp engine, supporting modern features like lookaheads, lookbehinds, named capture groups, and unicode property escapes.
How do capture group variables like $1 work in replacement?
In the replacement box, $1, $2, etc., reference numbered capture groups in parentheses within your pattern. $& represents the full match, and $
What happens if I write an invalid regex pattern?
If the expression contains a syntax error, an error banner immediately appears displaying the exact SyntaxError message from the JavaScript engine.