Regex Tester

A regular expression (regex or regexp) is a sequence of characters defining a search pattern. Developed from formal language theory in the 1950s and implemented in computing tools since the 1960s, regex is now supported in virtually every programming language. It's used for text search and matching, input validation (emails, phone numbers), data extraction (log parsing, scraping), and find-and-replace operations. The syntax may look cryptic at first, but it's an incredibly powerful tool once learned.

The g (global) flag finds all matches in the string rather than stopping at the first. The i flag makes matching case-insensitive so /hello/i matches "Hello", "HELLO", and "hello". The m (multiline) flag makes ^ and $ match at the start and end of each line, not just the whole string. The s (dotAll) flag makes . match newline characters (it normally doesn't). The u (unicode) flag enables full Unicode support, making . correctly match emoji and other multi-byte characters.

By default, quantifiers *, +, and {n,m} are greedy — they match as much text as possible. For example, <.*> applied to <a>text</a> matches the entire string from the first < to the last >. Add ? after any quantifier to make it lazy (non-greedy): <.*?> matches just <a> and </a> separately. Lazy quantifiers match the minimum possible text.

Parentheses () in a regex create capture groups that extract portions of the matched text. For example, (\d{4})-(\d{2})-(\d{2}) matching "2025-01-15" captures "2025" as group 1, "01" as group 2, and "15" as group 3. In replacements, you can reference groups with $1, $2, etc. Use (?:...) for non-capturing groups when you need grouping for alternation or quantifiers but don't need to extract the match.

No. All regex compilation, matching, and replacement happens entirely in your browser using JavaScript's built-in RegExp engine. Your patterns and test text never leave your device.