Regex Tester

Regex Tester is a free online tool that tests and debugs regular expressions against any input text in real time. Enter your pattern, select your flags, paste test text, and the tool instantly highlights every match and shows the captured groups for each one. Modify your pattern and see results update immediately — making it the fastest way to build and verify regular expressions without switching between your editor and a test environment.

A regular expression (regex or regexp) is a sequence of characters defining a search pattern. Regular expressions are used in virtually every programming language and text processing tool to search for, match, validate, and transform text. A regex can be as simple as a literal string ("cat" matches the word "cat") or as complex as a multi-line pattern with named groups, lookaheads, and Unicode property escapes. Regular expressions are among the most powerful tools available to developers — and also among the most error-prone to write correctly without live testing.

The most common regex use cases in web development include: form input validation for email addresses, phone numbers, postal codes, credit card numbers, and password strength rules; URL route matching in web frameworks; text extraction from API responses, log files, and structured text; find-and-replace operations in code editors and shell scripts; sanitising and normalising user-submitted text; and data transformation pipelines that convert one text format into another. Regex is available natively in JavaScript, Python, Java, Ruby, Go, PHP, and virtually every mainstream language.

Regex flags control how the engine interprets the pattern. The global flag (g) finds all matches instead of stopping at the first. The case-insensitive flag (i) makes matching ignore upper/lowercase. The multiline flag (m) makes ^ and $ match the start and end of each line rather than the entire string. The dotAll flag (s) makes the dot (.) match newline characters. The unicode flag (u) enables full Unicode matching. Our tester lets you toggle all standard flags and shows which matches each combination produces, making it easy to understand how each flag changes matching behaviour.

Capture groups are one of the most powerful regex features. Enclosing part of a pattern in parentheses captures that portion of the match for later reference or extraction. The pattern (\d{4})-(\d{2})-(\d{2}) matches a date and captures the year, month, and day in three separate numbered groups. Named capture groups (?<year>\d{4}) make references self-documenting and are accessible by name in replacement strings. Our tester displays all capture groups — both numbered and named — for every match alongside the full match text.

Developers use Regex Tester when writing input validation patterns — testing that an email pattern correctly rejects "user@" but accepts "user@example.com", or that a phone pattern handles international dialling codes. Data engineers write extraction patterns for parsing log files, structured text reports, and API response fields. Security professionals write SIEM detection rules and alert patterns using regex. Content writers use advanced find-and-replace in editors, and regex testers help verify their patterns before applying them to entire documents.

Regex Tester runs entirely in your browser using JavaScript's native RegExp engine. As you type your pattern and test text, the tool re-evaluates the regex in real time, highlighting all matches inline and displaying capture groups. Invalid regex syntax is reported immediately with an error message. No data is sent to any server. Note that JavaScript's regex flavour may differ slightly from PCRE (used in PHP and Python's re module) in advanced features like lookbehind length restrictions.