What is Regex Tester?
The Regex Tester is an interactive tool for writing, testing, and debugging regular expressions against sample text in real time. Regular expressions (regex) are a powerful pattern-matching language used across virtually every programming language and text editor for tasks like input validation, search-and-replace, log parsing, and data extraction. Despite their power, regex patterns can be notoriously difficult to write and understand, especially as they grow in complexity. This tool bridges that gap by providing instant visual feedback: matches are highlighted in the test string, capture groups are listed with their indices and values, and a plain-English explanation panel translates each part of your pattern into understandable language so you always know what your regex is doing.
Why Use Regex Tester?
Writing regex without immediate feedback is like coding blindfolded. A single misplaced quantifier or forgotten escape character can change the entire meaning of a pattern. This tool provides instant match highlighting as you type, so you can iterate on your pattern and test string simultaneously. The capture group panel makes it easy to verify that parenthetical groups are extracting the right substrings. The explanation panel demystifies cryptic syntax like (?<=...) (lookbehind) or \b (word boundary), making regex accessible to beginners while still being a productivity booster for experts. You can also toggle flags like global (g), case-insensitive (i), multiline (m), and dotall (s) with a single click.
How to Use
Enter your regular expression in the pattern field at the top. Type or paste your test string in the text area below. Matches are highlighted instantly as you type. Toggle regex flags (g, i, m, s, u) using the flag buttons. The matches panel lists every match with its index, value, and any capture groups. The explanation panel provides a step-by-step breakdown of the pattern in plain English. You can also use the substitution field to test replace operations — enter a replacement string with back-references ($1, $2) and see the transformed output live. Save useful patterns to your local library for later reuse.
Example Usage
Say you need to extract email addresses from a document. Enter the pattern: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}. Paste your text containing mixed content, and the tool highlights every email match in context. The explanation panel tells you that [a-zA-Z0-9._%+-]+ matches 'one or more alphanumeric characters, dots, underscores, percent signs, plus signs, or hyphens' followed by a literal '@', then the domain. If you add a capture group around the domain — @([a-zA-Z0-9.-]+\.[a-zA-Z]{2,}) — the matches panel shows the domain separately, which is useful for data extraction pipelines.
Benefits
Real-time match highlighting removes the trial-and-error cycle of testing regex. The plain-English explanation panel is both a learning tool and a documentation aid. Capture group extraction makes it easy to build and verify complex data-parsing patterns. Support for all JavaScript regex flags ensures compatibility with your code. The substitution preview lets you test search-and-replace patterns without modifying source files. Local pattern library keeps your commonly used regex patterns a click away.