Regex Tester & Visualizer - Test Regular Expressions Online

A JavaScript regex engine is used, which supports most standard regex features including lookahead, lookbehind, and Unicode properties.

A g = global (all matches), i = case-insensitive, m = multiline (^ and $ match line boundaries), s = dotAll (. matches newlines), u = Unicode mode.

A Yes, paste multiline text into the test area and enable the m flag for line-based matching.

A Yes, this Regex Visualizer is ideal for developing and testing robust form validation patterns. You can easily input your specific regex, paste various valid and invalid test strings, and instantly see the matches and capture groups. The tool's built-in presets for common patterns like email addresses, URLs, and IP addresses also provide excellent examples, helping you ensure your web forms handle user input correctly.

A This Regex Visualizer clearly highlights capture groups within your test string. After entering your regex with parentheses, you'll see matched portions of text visually distinct. Below the input areas, a dedicated section displays each capture group's content for every match, letting you easily inspect exactly what data your regular expression is successfully extracting. This is invaluable for data parsing and extraction tasks.

A You can use the built-in presets. Just click the dropdown menu labeled 'Presets'. Then, select an option like 'IPv4 Address' or 'HTTP URL'. The tool automatically populates the regex input field with a tested pattern. It's a huge time-saver, especially for freelance developers building client websites.

A Check if the 'g' flag is enabled. Without it, the engine stops after the first match, which can create false positives in validation. For example, testing /\d+/ against "abc123def" without 'g' shows one match, but with 'g' it shows only that single match too. The real issue is often missing anchors. Adding ^ at the start and $ at the end forces full-string matching. Try toggling flags one at a time to isolate what's causing unexpected behavior.

A Yes, lookbehind assertions work here. You can use both fixed-length (?<=pattern) and (?<!pattern) forms. Variable-length lookbehinds like (?<=ab+) also work in this JavaScript-based engine. Just paste a pattern like (?<=@)\w+ to extract usernames from emails. One tip: if your lookbehind isn't matching, check if the 'u' flag is enabled — some Unicode patterns need it.