Regex Tester
Test and debug regular expressions in real-time with instant match highlighting, capture groups, and find-and-replace.
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Regular Expression
Test String
Replace
Match Details
Enter a regex pattern to see matches.
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How to Use the Regex Tester
Our free online regex tester helps you write, test, and debug regular expressions in real-time. Whether you're validating user input, extracting data from log files, or building complex search patterns, this tool provides instant visual feedback as you type.
To get started, enter your regular expression pattern in the pattern field. You can type any valid JavaScript regex pattern — from simple character matches like\d+ to complex patterns with lookaheads and named groups. The tool validates your pattern instantly and shows any syntax errors.
Next, paste or type your test string in the text area below. As you type, the tool highlights all matches directly in the text. Each match is color-coded so you can see exactly what your pattern captures. The match details section shows the full match text, its position (index) in the string, and any capture groups.
Use the flag toggles to control how the regex engine processes your pattern. The global (g) flag is enabled by default so you see all matches. Toggle case insensitive (i) if capitalization doesn't matter for your use case, or enable multiline (m) when working with text that spans multiple lines.
The replace feature lets you test search-and-replace operations. Enable it, enter a replacement string (using $1, $2, etc. for capture group references), and see the result instantly. This is invaluable for testing transformations before using them in your code.
Need inspiration? Click any of the preset patterns — email, URL, phone number, and more — to load a working regex pattern instantly. All processing happens entirely in your browser with zero server requests, keeping your data private and the tool lightning-fast.
Frequently Asked Questions
What is a regular expression (regex)?
A regular expression (regex) is a sequence of characters that defines a search pattern. It is used in programming and text processing for matching, searching, and replacing text. Common uses include form validation, data extraction, and search-and-replace operations in code editors.
What do the regex flags mean?
Regex flags modify how the pattern is applied. The 'g' (global) flag finds all matches instead of stopping at the first. The 'i' flag makes matching case-insensitive. The 'm' (multiline) flag makes ^ and $ match the start and end of each line. The 's' (dotAll) flag makes the dot (.) match newline characters. The 'u' (unicode) flag enables full Unicode matching.
How do capture groups work in regex?
Capture groups are created by wrapping part of a pattern in parentheses, like (\d+). When a match is found, the text matched by each group is captured separately and can be referenced by its number ($1, $2, etc.) in replacements. Named capture groups use the syntax (?<name>pattern) and can be referenced by name.
Is my text processed on a server?
No. This regex tester runs entirely in your browser using JavaScript's built-in RegExp engine. Your text never leaves your device — there are no server requests. This makes it fast, private, and works offline.
What regex flavor does this tester use?
This tester uses JavaScript's native RegExp engine, which follows the ECMAScript specification. It supports features like lookahead, lookbehind (in modern browsers), named capture groups, and Unicode property escapes. Most patterns that work here will work in JavaScript, TypeScript, and Node.js applications.