Regex Tester
Test regular expressions against sample text with live highlighting and detailed match information.
//
g
Flags:
Pattern Presets:
Test String Presets:
Match Results
Quick Reference
Character Classes
| \d | Digit (0-9) |
| \w | Word character (a-z, A-Z, 0-9, _) |
| \s | Whitespace |
| . | Any character except newline |
Quantifiers
| * | 0 or more |
| + | 1 or more |
| ? | 0 or 1 |
| {n} | Exactly n times |
| {n,m} | Between n and m times |
Related Tools
About Regex Tester
How Regex Tester Works
- Enter a regular expression pattern to test
- Choose appropriate flags (global, case-insensitive, etc.)
- Enter or select a test string to match against
- Matches are highlighted automatically in real-time
- Test replacement patterns with capture groups
- View detailed match information and positions
Common Use Cases
- Validate email addresses, URLs, phone numbers
- Extract data from structured text
- Find patterns in log files or code
- Test search and replace operations
- Learn regex through interactive experimentation
- Verify patterns before implementing in code
Frequently Asked Questions
What is a regular expression (regex)?
A regular expression is a sequence of characters that defines a search pattern, mainly used for pattern matching with strings. Regular expressions are powerful tools for text processing and are supported in most programming languages.
How do I use the Regex Tester tool?
Enter your regex pattern in the top input field, select appropriate flags, then enter your test string in the text area below. The tool will automatically highlight any matches in real-time and display detailed information about each match found.
What do the different regex flags mean?
Regex flags modify how pattern matching works: (g) Global - find all matches rather than stopping at the first match. (i) Ignore Case - case-insensitive matching. (m) Multiline - ^ and $ match start/end of each line. (s) Dot All - dot (.) matches newlines too. (u) Unicode - enable full Unicode matching. (y) Sticky - match only from lastIndex position.
How do I use the replacement feature?
Enter your replacement pattern in the "Replacement Text" field. You can use $1, $2, etc., to reference captured groups from your regex pattern. Click the "Replace" button to see the result of replacing all matches with your specified text.
What are the pattern presets available in this tool?
The tool includes common pattern presets like email validation, URL matching, phone numbers, dates, IP addresses, HTML tags, password validation rules, and zip codes. Click on any preset button to instantly load that pattern for testing.
Is my data safe when using this Regex Tester?
Yes, all processing occurs entirely in your browser. No data is sent to our servers. You can use the tool confidently with sensitive text as nothing leaves your computer.
Why am I getting an "excessive backtracking" error?
This error occurs when a regex pattern causes too many recursive steps, which can lead to performance issues. This often happens with nested quantifiers like (a+)+ on certain inputs. Try simplifying your pattern or making quantifiers more specific.
Can I use this Regex Tester for different programming languages?
This tool uses JavaScript's RegExp engine, which is similar to many other programming languages but may have some differences in advanced features. Most basic regex patterns work similarly across languages, but language-specific features might vary.
What are capture groups and how do I use them?
Capture groups are portions of a pattern enclosed in parentheses () that "capture" the matched text for later use. In replacement patterns, you can reference these groups with $1, $2, etc. (e.g., with pattern (\w+)@(\w+\.\w+), replacement "$2 ID: $1" would transform "user@example.com" to "example.com ID: user").
How can I learn more about regular expressions?
The tool includes a quick reference guide at the bottom of the page. For more comprehensive learning, we recommend checking out resources like MDN Web Docs, RegexOne.com for interactive tutorials, or books like "Mastering Regular Expressions" by Jeffrey Friedl.