Regex101 is an online tool for writing and testing regular expressions. It provides real-time matching, syntax highlighting, and a detailed explanation panel that breaks down what each part of the pattern does. You can choose the regex flavor (PCRE, JavaScript, Python, Go, and others), which matters because behavior differs between languages. A debugger helps trace why a pattern matches or fails.

Developers use Regex101 to build patterns incrementally, test edge cases, and share working examples with teammates. Saved regexes get a unique URL that can be pasted into documentation or bug reports. For anyone who works with text processing, log parsing, or form validation, it turns one of programming's more frustrating tasks into something manageable.