Regex Tester

Test your regular expressions with sample text. This tool processes all data locally in your browser. No information is ever sent to any server. Completely free, no registration required.

How to Use the Regex Tester

1. Enter your input values above
2. Results update automatically
3. Copy or download the output

What is a Regex Tester?

A Regex Tester lets you write, test, and debug regular expressions in real time against sample text, with match highlighting, capture group extraction, and detailed explanations. Regex is one of the most powerful yet frustrating tools in a developer's toolkit — a single well-crafted regex can replace dozens of lines of string manipulation code, but even experienced developers struggle to write correct patterns on the first try. This tester provides instant visual feedback: matches light up as you type, unmatched parts are clearly shown, and every capture group is labeled.

How Does It Work?

Enter your regex pattern and test text. As you type, the tester highlights all matches in real time. Below the match results, captured groups are listed by number and name. The explainer breaks down each component of your regex in plain English: /^(\d{3})-(\d{2})-(\d{4})$/ → 'start of string, capture group 1: exactly 3 digits, literal hyphen, capture group 2: exactly 2 digits, literal hyphen, capture group 3: exactly 4 digits, end of string.' Supported flags: g (global), i (case insensitive), m (multiline), s (dotall).

Formula

Common Regex Tokens:\n• ^ start, $ end, . any char, * 0+, + 1+, ? 0 or 1\n• {n} exactly n, {n,} n+, {n,m} n to m\n• [abc] character class, [^abc] negated class\n• \\d digit, \\w word, \\s whitespace, \\b word boundary\n• (abc) capture group, (?:abc) non-capturing\n• (?<name>abc) named capture group\n• | alternation (OR)\n• (?=...) lookahead, (?<=...) lookbehind\n\nFlags: /g global, /i case-insensitive, /m multiline (^$ match line), /s dotall (. matches \\n)\n\nEmail Regex: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$/\nURL Regex: /^https?:\\/\\/(www\\.)?[-a-zA-Z0-9@:%._+~#=]{2,256}\\.[a-z]{2,6}\\b/

Who Uses This Tool?

• Web developers writing form validation regex for emails, phones, URLs
• Data analysts extracting structured data from logs and text files
• Backend developers parsing request parameters and URL routings
• Security engineers writing input sanitization and WAF rules
• Anyone who's ever asked Stack Overflow 'regex for X'

Pro Tips

• Start simple and build up: write the smallest regex that matches, test it, add complexity incrementally. Don't try to write the perfect regex in one shot
• Use online regex communities (regex101.com, RegExr) for complex patterns. The collective regex wisdom on the internet has solved most common problems already
• For email validation: don't over-constrain. The only truly accurate regex for RFC 5322 is ~6,000+ characters. Instead, use /.*@.*\\..*/ and send a verification email
• Capture groups slow down regex engines. Use non-capturing groups (?:) when you don't need the extracted value. In performance-critical code, avoid unnecessary groups

Frequently Asked Questions about Regex Tester

What's the difference between greedy and lazy matching?

By default, quantifiers (*, +, ?) are greedy — they match as much as possible. Adding ? makes them lazy — they match as little as possible. 'Hello World' with /H.*o/ matches 'Hello Wo' (greedy, stops at last o). /H.*?o/ matches 'Hello' (lazy, stops at first o).

Why does my regex work on the tester but not in my code?

Different languages support different regex features. JavaScript doesn't support lookbehinds in older browsers. Python needs raw strings r'...'. PHP uses PCRE which has different escape rules. Always test in the same engine/environment as your production code.

Free online Regex Tester — no signup, 100% client-side processing. All data stays in your browser.