Regex Tester

It uses JavaScript (ECMAScript) regular expressions, which is the same engine used in all modern browsers, Node.js, and Deno. Most common regex features are supported including lookaheads, lookbehinds, named groups, and Unicode properties.

g (global) finds all matches. i (case-insensitive) ignores case. m (multiline) makes ^ and $ match line boundaries. s (dotAll) makes . match newlines. u (unicode) enables Unicode matching.

No. All matching runs locally in your browser.

JavaScript regex is similar to PCRE but has some differences: it does not support possessive quantifiers or atomic groups, and lookbehind assertions (added in ES2018) must be fixed-width in older engines. Python's re module uses PCRE-like syntax and supports verbose mode (re.VERBOSE) for commented patterns. For cross-language compatibility, stick to basic character classes, quantifiers, and groups.

Yes. Certain patterns with nested quantifiers (e.g., (a+)+ or (a|aa)+) can trigger catastrophic backtracking, causing the browser to hang. This is known as ReDoS (Regular Expression Denial of Service). If your regex is slow, look for nested quantifiers or overlapping alternatives and simplify them.

Email (basic): ^[\w.-]+@[\w.-]+\.[a-z]{2,}$. URL: https?:\/\/[\w.-]+(?:\/[\S]*)?. Date (YYYY-MM-DD): \d{4}-\d{2}-\d{2}. IP address: (\d{1,3}\.){3}\d{1,3}. Hex color: #[0-9a-fA-F]{3,6}. These are starting points -- always test with edge cases.

Yes. The interface is responsive. You can enter patterns, paste test text, and see live matches on any phone or tablet. The match highlighting and group captures display correctly on small screens.

regex101.com supports multiple flavors (PCRE, Python, Go) and has a detailed debugger. This tool is focused on JavaScript regex with instant highlighting, a pattern library, and zero distractions. It also runs fully offline once loaded — no server round-trips.

It breaks your pattern into individual tokens and describes each one in plain English. For example, \d{2,4} becomes "digit, 2 to 4 times". This helps you understand complex patterns written by others or verify that your own pattern does what you intend.

Yes. JavaScript supports named groups with the syntax (?<name>pattern). For example, (?<year>\d{4})-(?<month>\d{2}) captures year and month as named groups. The match results panel shows both numbered and named group values.

Enable the s (dotAll) flag to make . match newline characters, and the m (multiline) flag to make ^ and $ match at line boundaries instead of just the start/end of the entire string. Both flags can be toggled in the flag checkboxes.

The <a href="/ai/dev/case-converter">Case Converter</a> helps format matched text into programming naming conventions. The <a href="/ai/dev/diff">Text Diff Checker</a> can compare text before and after regex-based replacements. The <a href="/ai/dev/url-encode">URL Encoder</a> is useful when testing regex patterns against URL-encoded strings.