What is Regex?
A plain-English definition, and the tools that work with it.
A regex (regular expression) is a compact pattern that describes and matches text, for example, all email addresses or every date in a document. Developers use it to search, validate and replace text with precision.
- Regex is expressive but easy to get subtly wrong, so testing it against real sample text is essential.
- Small syntax differences exist between languages (JavaScript, Python, PCRE), a pattern isn't always portable.
- Common uses: validating input (emails, phone numbers), find-and-replace, and extracting structured bits from messy text.
Tools for Regex
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Regex Tester
Build and debug regular expressions with live match highlighting. Toggle the g/i/m/s/u/y flags, watch every match light up in your test string, inspect numbered and named capture groups, preview a find-and-replace, and drop in ready-made patterns from a built-in library. 100% in-browser. Nothing is uploaded.
Diff Checker
Compare two texts side by side and highlight every added, removed and changed line, word or character, with unified or split view, ignore-whitespace and ignore-case, all in your browser.
JSON Validator
Validate JSON against RFC 8259 with the exact error line, column and a caret under the bad character. Get plain-English explanations, one-click auto-fix for trailing commas and single quotes, plus depth, key-count and type stats, nothing uploaded.
URL Parser
Paste any URL and instantly break it into protocol, host, port, path, every query parameter, hash and origin, with a live rebuilt URL to confirm the parts. 100% in your browser.
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