What are Regular Expressions (Regex)?
A Regular Expression (commonly known as Regex or RegExp) is a powerful sequence of characters that forms a search pattern. These patterns are widely used by developers for string searching, validation (like checking if an email is formatted correctly), replacing text, and data extraction within programming languages like JavaScript, Python, and PHP.
How to Test Regex Patterns
Writing regular expressions can be notoriously difficult to get right on the first try. Our online Regex Tester helps you debug your patterns interactively. As you type your regex in the top field, the tool instantly evaluates it against your test string and highlights all matches in real-time. If your pattern includes Capture Groups (using parentheses), the tool will extract and display those groups in a structured list below the text area.
Supported Regex Flags
Regular expressions can be modified using "flags" that change how the search is performed. Our tester supports the standard JavaScript regex flags:
- Global (g): Don't return after the first match. Find all matches in the text.
- Case Insensitive (i): Ignore case differences (e.g., matching 'a' and 'A' identically).
- Multiline (m): Changes the behavior of
^and$to match the start and end of individual lines within a multi-line string, rather than just the start and end of the entire string. - Dotall (s): Allows the dot character (
.) to match newline characters.
Frequently Asked Questions (FAQ)
Which Regex engine does this tool use?
Because this tool runs in your web browser, it relies on the JavaScript RegExp engine (ECMAScript flavor). While regex syntax is mostly standardized, there are slight differences between JS, PCRE (PHP), and Python. Most common patterns will work identically across all engines.
Is my test data uploaded anywhere?
No. Your regex patterns and test strings are evaluated locally using your browser's JavaScript engine. We do not store, track, or upload any of the data you paste into the tester, making it safe for debugging production logs or sensitive user data.
What are Capture Groups?
Capture groups are created by wrapping part of your regex in parentheses ( ). They allow you to extract specific parts of a matched string. For example, in the regex (\d{4})-(\d{2})-(\d{2}), you are not just matching a date, but "capturing" the year, month, and day separately. Our tool visualizes these groups so you can verify your extractions.