About the Regex Tester
The Regex Tester lets you write a regular expression, run it against sample text and see every match and capture group immediately. Regular expressions are compact patterns for finding and validating text: email addresses, phone numbers, dates, log lines, URLs and much more. Testing a pattern on real examples before you put it in code saves a lot of debugging time and prevents subtle bugs in form validation and data parsing.
How to Use the Regex Tester
- Type your pattern in the regex field, for example
\d{4}-\d{2}-\d{2}for dates. - Paste the text you want to search in the test string box.
- Choose options such as case-insensitive or multiline if you need them.
- Run the test and review the list of matches and capture groups.
- Adjust the pattern until it matches what you want and nothing else, then copy it into your code.
Regex Cheat Sheet
. any character, \d digit, \w letter, digit or underscore, \s whitespace. * zero or more, + one or more, ? optional, {2,5} between two and five times. ^ start and $ end of the line. [abc] any of a, b or c; [^abc] anything except them. ( ) creates a capture group, (?: ) groups without capturing, and a|b means a or b. Escape special characters with a backslash, for example \. for a literal dot.
Useful Example Patterns
Pakistani mobile number: ^03\d{2}-?\d{7}$. Simple email check: ^[^@\s]+@[^@\s]+\.[^@\s]+$. Hex color: ^#?([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$. ISO date: ^\d{4}-\d{2}-\d{2}$. Extract numbers from text: -?\d+(\.\d+)?. Always test these against both valid and invalid samples.
Avoiding Common Mistakes
Forgetting ^ and $ means a validation pattern can match part of a bad string. Greedy quantifiers like .* can match far more than you expect; use the lazy form .*? when extracting text between delimiters. Nested quantifiers such as (a+)+ can cause very slow matching on long inputs, so keep patterns simple. Remember that some regex features differ between JavaScript, .NET, Python and PHP.
Frequently Asked Questions
Which regex flavor does the tester use?
Patterns are evaluated with the .NET regular expression engine. Most everyday syntax is identical in JavaScript, Python and PHP, but advanced features like look-behind can differ slightly.
What is a capture group?
Anything inside parentheses is captured separately, so you can extract parts of a match, such as the year, month and day from a date.
Why does my pattern match too much?
Quantifiers are greedy by default. Add ? after them (for example .*?) or use a more specific character class.
Should I validate emails with regex?
A simple regex is fine for catching typos, but the only reliable proof that an email exists is sending a confirmation message.