Test and debug regular expressions online with real-time match highlighting, group extraction, and flag support. Free regex tester that works instantly in your browser.
Regex Tester is a free online tool that lets you write, test, and debug regular expressions in real time against any test string. It supports all standard JavaScript regex flags including global (g), case-insensitive (i), multiline (m), and dotAll (s), giving you full control over matching behavior. As you type your pattern and test input, matches are highlighted and listed instantly, providing immediate visual feedback. Everything runs locally in your browser — no data is ever sent to a server, so you can safely test patterns against sensitive text.
Use the Regex Tester whenever you're building or refining a regular expression pattern and want to validate it against sample data before integrating it into your code. It's especially valuable during form validation development, log parsing, data extraction tasks, or when you're writing patterns for search-and-replace operations. The real-time feedback loop helps you catch edge cases and off-by-one matching errors much faster than testing within your application code.
Developers and data engineers frequently use Regex Tester to craft and validate email, phone number, and URL validation patterns, build expressions for parsing log files and extracting structured data from unstructured text, develop search patterns for code refactoring using find-and-replace workflows, create patterns for input sanitization and security filtering, and prototype data extraction rules for web scraping or ETL pipelines. It's also a great learning tool for understanding how regex quantifiers, groups, lookaheads, and character classes behave.
Essential terms and definitions related to Regex Tester.
A sequence of characters that defines a search pattern used for matching, finding, and replacing text. Regex is supported in virtually all programming languages and text editors. Patterns use special metacharacters like . (any character), * (zero or more), + (one or more), and [] (character class) to describe complex matching rules.
A portion of a regex pattern enclosed in parentheses () that extracts and remembers the matched substring. Capture groups allow you to isolate specific parts of a match — for example, (\d{4})-(\d{2})-(\d{2}) captures year, month, and day separately from a date string. Groups are numbered from left to right and can be referenced in replacements.
Modifiers appended to a regex pattern that change its matching behavior. Common flags: g (global — find all matches, not just the first), i (case-insensitive), m (multiline — ^ and $ match line boundaries), s (dotAll — . matches newlines), and u (unicode — enables Unicode property escapes and correct surrogate pair handling).
Zero-width assertions that check whether a pattern exists ahead of or behind the current position without consuming characters. Lookahead (?=...) matches if the pattern follows; negative lookahead (?!...) matches if it does not. Lookbehind (?<=...) and negative lookbehind (?<!...) work in the reverse direction. They are essential for complex validation and extraction patterns.
This tool uses JavaScript regular expressions, which follow the ECMAScript specification. Most common regex features are supported including lookahead, lookbehind (in modern browsers), character classes, and quantifiers.
Yes. The tool supports all standard JavaScript regex flags including g (global), i (case-insensitive), m (multiline), s (dotAll), and u (unicode). You can toggle these flags in the interface.
Yes. When your regex contains capture groups using parentheses, the tool displays each matched group separately. This makes it easy to debug complex patterns and verify that your groups capture the expected content.
Different programming languages implement different regex flavors. JavaScript regex lacks some features found in PCRE (PHP, Python) such as possessive quantifiers and recursive patterns. Always test your regex in the target language environment for final validation.
Common errors developers encounter and how to resolve them.
Catastrophic Backtracking: Regex freezes the browser Patterns containing nested quantifiers (e.g., (a+)+ or (a|b)*c) can cause exponential time complexity on long non-matching inputs — this is known as "catastrophic backtracking" and is the basis of ReDoS (Regular Expression Denial of Service) attacks. Solution: avoid nested quantifiers, use atomic grouping or possessive quantifiers (not available in JS — restructure the pattern instead). Use this tool with short test strings to detect backtracking issues in your patterns.
Lookbehind assertion not supported: (?<=...) error JavaScript lookbehind (?<=...) and negative lookbehind (?<!...) assertions were introduced in ES2018 and now work in all modern browsers including Safari 16.4+. However, older browsers (IE, older Safari) will throw a SyntaxError. Lookahead (?=...) is universally supported across all browsers. Check your target audience's browser compatibility, or alternatively use a capture group and filter the result with post-processing.
Unicode character matching fails: Non-ASCII characters and \w pattern In JavaScript regex, \w only matches [A-Za-z0-9_] and does not include non-ASCII characters (accented letters, CJK characters, etc.). To match Unicode characters, use the u (unicode) flag with Unicode property escapes: \p{Letter} or \p{Script=Latin}. For example, /\p{L}+/gu matches letters from all languages. Enable the u flag in this tool to test your Unicode patterns.
Email validation regex rejects valid addresses There is no single "perfect" regex for email validation — the RFC 5322 specification is extremely complex. Simple patterns (^[a-z]+@[a-z]+\\.[a-z]+$) reject many valid addresses: plus signs (user+tag@), subdomains, and internationalized domain names (IDN). Practical solution: use a broad pattern like /^[^\s@]+@[^\s@]+\.[^\s@]+$/ and perform real validation by sending a confirmation email. Test your pattern against various email formats using this tool to verify its coverage.