Regex Tester & Builder

What is a Regex Tester?

A regex tester is a tool that allows you to test and validate regular expressions (regex) in real-time. It helps you develop and debug patterns before using them in production code. This tool provides instant feedback on matches, capture groups, and allows you to test replacements.

How to Use the Regex Tester

1. Enter a Pattern: Type your regular expression in the pattern field. Start simple and build complexity.
2. Set Flags: Click the flag badges to enable global (g), case-insensitive (i), multiline (m), dotall (s), or unicode (u) matching.
3. Enter Test String: Paste or type the text you want to test against your pattern.
4. View Results: Matches are highlighted in the results area. The match count and details appear instantly.
5. Test Replacement: Enter replacement text and click "Replace All" to see how your pattern substitutes matches.
6. Use Quick Patterns: Select a common pattern from the dropdown to get started quickly.

Common Regular Expression Use Cases

• Email Validation: Verify email addresses match a valid format
• URL Extraction: Find and extract URLs from text
• Phone Numbers: Match and format phone numbers
• Date Parsing: Extract and validate dates in specific formats
• Text Replacement: Find and replace patterns in large texts
• Data Extraction: Pull specific data from unstructured text
• Form Validation: Validate user input in web forms

Understanding Regex Flags

• g (Global): Finds all matches instead of stopping at the first match
• i (Case-insensitive): Ignores uppercase/lowercase differences
• m (Multiline): Makes ^ and $ match line boundaries, not just string boundaries
• s (Dotall): Makes . match newline characters as well
• u (Unicode): Enables full Unicode support in patterns

Capture Groups and Backreferences

Parentheses in regex create capture groups that can be referenced in replacements. For example, the pattern (\w+) (\w+) captures two words, and in the replacement field you can use $2 $1 to swap them.

Frequently Asked Questions

What's the difference between . and \s?

. matches any character except newlines. \s matches only whitespace characters (spaces, tabs, newlines). Use . for general matching and \s when you specifically need whitespace.

How do I match a literal dot or special character?

Escape special characters with a backslash. For example, use \. to match a literal dot, \\ for a backslash, or \$ for a dollar sign.

What's the difference between + and *?

+ requires at least one occurrence of the preceding element, while * allows zero or more. For example, a+ matches "a", "aa", "aaa" but not an empty string.

How do I match newlines?

Use \n for newlines, \r for carriage returns, or \s to match any whitespace. Enable the multiline (m) flag to make ^ and $ match line boundaries.

Can I use JavaScript replace() method syntax?

Yes! This tool supports backreferences like $1, $2 for capture groups, and $& for the entire match.