Test Regular Expressions Online

What Are Regular Expressions?

Regular expressions (regex or regexp) are powerful pattern-matching sequences used across nearly every programming language, text editor, and command-line tool. At their core, they let you describe what a string should look like rather than specifying exact characters. A single regex pattern can match thousands of different strings -- for example, the pattern \d{3}-\d{3}-\d{4} matches any US phone number formatted as 555-123-4567, regardless of the actual digits. This makes regex indispensable for data validation, text parsing, search-and-replace operations, and web scraping.

Common Patterns Every Developer Should Know

While regex syntax can become complex, a handful of core patterns cover most real-world needs. Email validation often uses [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}, though production email validation should use a library for full RFC compliance. URL matching typically starts with https?://[^\s]+. For extracting numbers from text, \d+(\.\d+)? captures both integers and decimals. Learning character classes (\d, \w, \s), quantifiers (+, *, ?, {n,m}), and anchors (^, $, \b) gives you enough building blocks to tackle the vast majority of text processing tasks.

Tips for Testing and Debugging Regex

The most common mistake in regex is over-matching or under-matching due to greedy quantifiers. By default, .* is greedy and consumes as much text as possible; adding a ? makes it lazy (.*?), matching the shortest possible string. Always test your patterns against edge cases: empty strings, strings with special characters, and strings where the pattern should NOT match. This online tester runs entirely in your browser, so your data is never transmitted to a server -- making it safe to test patterns against sensitive text. Use the global flag to find all matches, and pay attention to capture groups to ensure you are extracting exactly the data you need.