What is the difference between the 'g' and 'i' flags?

The 'g' (global) flag finds all matches in the string rather than stopping at the first match. The 'i' (case-insensitive) flag makes the pattern match regardless of letter casing, so 'abc' would match 'ABC', 'Abc', and any other case combination.

How do capture groups work in regular expressions?

Capture groups are created by wrapping part of a pattern in parentheses, like (\d+). They extract specific portions of a match for later use. In replacements, you reference captured groups with $1 for the first group, $2 for the second, and so on. Non-capture groups (?:...) group patterns without capturing.

Why does my regex match differently than expected?

Common issues include greedy vs. lazy quantifiers (e.g., .* vs. .*?), missing escape characters for special symbols like dots or brackets, and forgetting to enable the global flag for multiple matches. Check the cheat sheet for proper syntax and test incrementally.

Can I use lookahead and lookbehind assertions?

Yes, this tool supports JavaScript-compatible regex syntax including positive lookahead (?=...), negative lookahead (?!...), positive lookbehind (?<=...), and negative lookbehind (?<!...). These are zero-width assertions that match positions without consuming characters.

Is my data processed securely when using this tool?

Absolutely. All regex processing happens entirely in your browser using JavaScript's built-in RegExp engine. No data is sent to any server, ensuring your text and patterns remain completely private and secure on your local machine.

Regex Tester - Regular Expression Test & Replace | Free Online Tool

Regex Pattern

//g

gims

Test String

Replace

Common Patterns

Cheatsheet

Character Classes

. - Any character

\d - Digit [0-9]

\w - Word char [a-zA-Z0-9_]

\s - Whitespace

Quantifiers

* - 0 or more

+ - 1 or more

? - 0 or 1

{n} - Exactly n times

{n,m} - n to m times

Anchors

^ - Start of string

$ - End of string

\b - Word boundary

Groups

(abc) - Capture group

(?:abc) - Non-capture group

a|b - OR operation

What is Regex Tester?

Regex Tester is an interactive online tool that allows you to write, test, and debug regular expressions in real time. Regular expressions (regex) are powerful pattern-matching sequences used in programming, text processing, and data validation. This tool provides instant visual feedback by highlighting matches, displaying capture groups, and supporting string replacement — making it an indispensable companion for developers, data analysts, and anyone working with text patterns.

Real-time pattern matching with instant visual highlighting of all matches
Support for regex flags including global, case-insensitive, and multiline modes
Capture group extraction and detailed match information display
Find-and-replace functionality with group reference support ($1, $2)
Built-in cheat sheet with common patterns, quantifiers, and character classes
Pre-built common patterns for email, URL, date, IP address, and more

How to Use

1
Enter Your Regex Pattern
Type your regular expression in the pattern input field. The tool accepts standard regex syntax including character classes, quantifiers, anchors, and groups.
2
Set Flags
Toggle the regex flags you need: 'g' for global matching (find all occurrences), 'i' for case-insensitive matching, and 'm' for multiline mode where ^ and $ match line boundaries.
3
Enter Test String
Paste or type the text you want to test against your pattern. Matches will be highlighted immediately as you type, giving you instant feedback on your regex.
4
Review Match Results
Examine the match details section to see all matched substrings, their positions, and any captured groups. This helps you verify that your pattern captures exactly what you intend.
5
Use Replace (Optional)
Enter a replacement string to see how your regex substitution will transform the text. Use $1, $2, etc. to reference captured groups in the replacement.

Frequently Asked Questions

Tips & Best Practices

Start with a simple pattern and gradually add complexity — test each addition to ensure it works as expected before moving on.
Use non-greedy quantifiers (*?, +?, ??) when you want the shortest possible match rather than the longest.
Escape special regex characters (. * + ? ^ $ { } [ ] ( ) | \) with a backslash when you want to match them literally.
Use the built-in common patterns as starting points and customize them for your specific needs rather than writing from scratch.
Take advantage of character classes like \d (digits), \w (word characters), and \s (whitespace) for cleaner, more readable patterns.
Test your regex with edge cases including empty strings, special characters, and very long inputs to ensure robustness.