Free Regex Tester & Debugger - Test Regular Expressions Online

Regular expressions (regex) are patterns used to match, search, and manipulate text. Common uses include validating input (emails, phones, passwords), extracting data from text, search-and-replace operations, URL routing, and parsing log files. They're supported in almost every programming language and are essential for text processing tasks.

Different programming languages implement regex engines differently, leading to slight variations in syntax and features. For example, JavaScript doesn't support lookbehind in older versions, Python uses different flags, and PCRE has unique features. Our tool supports major flavors so you can test patterns for your specific language and avoid surprises in production.

Common regex flags: g (global) finds all matches instead of just the first; i (case-insensitive) ignores letter case; m (multiline) makes ^ and $ match line beginnings/ends; s (dotall/singleline) makes . match newlines. Understanding flags is crucial - the same pattern with different flags produces completely different results.

Use parentheses () to create capturing groups that extract matched portions. For example, (\d{3})-(\d{3})-(\d{4}) captures phone number parts separately. Our tool highlights each group in a different color and shows captured values. Use (?:...) for non-capturing groups when you need grouping without capturing.

Common issues: forgetting to escape special characters (. * + ? etc.), wrong flags (pattern is case-sensitive but text isn't), anchors (^ $) too restrictive, or testing against the wrong regex flavor. Our tool shows explanations and highlights matches in real-time, making it much easier to debug and fix patterns that aren't working as expected.

Never! All regex testing happens entirely in your browser using JavaScript's built-in regex engine. Your test strings, patterns, and data are processed locally and never transmitted to any server. This makes it completely safe to test with real customer data, log files, or sensitive text without privacy concerns.