Regex Tester & Validator

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regular expressions are one of the most powerful tools in programming for working with text. They let you search for patterns in strings, validate user input, extract structured data from unstructured text, and perform complex find-and-replace operations — all with a single expression.

Regular expressions are supported in virtually every programming language including JavaScript, Python, Java, C#, Go, Ruby, PHP, and Perl, as well as in command-line tools like grep, sed, and awk. While the syntax varies slightly between flavors, the core concepts — character classes, quantifiers, groups, and anchors — are universal.

The concept of regular expressions originates from formal language theory developed by mathematician Stephen Kleene in the 1950s. Ken Thompson brought them into computing by implementing them in the QED text editor and later in Unix tools. Today, regex is an indispensable skill for developers, data scientists, system administrators, and anyone who works with text data at scale.

At its core, a regex engine reads a pattern character by character and tries to match it against an input string. The engine walks through the string one position at a time, attempting to find a sequence of characters that satisfies the entire pattern. If a match attempt fails at any point, the engine backtracks and tries the next starting position.

Character Matching

Literal characters in a pattern match themselves. The pattern cat matches the exact string "cat" within any text. Special characters (called metacharacters) have special meanings: . matches any character, \d matches any digit, and \w matches any word character (letter, digit, or underscore).

Quantifiers

Quantifiers specify how many times a character or group should repeat. * means zero or more, + means one or more, ? means zero or one, and {n,m} means between n and m times. By default, quantifiers are greedy — they match as much as possible. Adding ? after a quantifier makes it lazy, matching as little as possible.

Groups and Capture

Parentheses () create capture groups that let you extract specific parts of a match. For example, (\d{4})-(\d{2})-(\d{2}) applied to "2024-03-15" captures "2024", "03", and "15" as separate groups. Named groups like (?<year>\d{4}) make your patterns more readable and maintainable.

Anchors

^ asserts the position at the start of the string (or line in multiline mode), while $ asserts the end. \b marks a word boundary — the position between a word character and a non-word character. These anchors don't match characters themselves but rather positions in the string.

Flags (also called modifiers) change how the regex engine interprets and applies a pattern. In JavaScript, flags are appended after the closing delimiter of the regex literal (/pattern/flags) or passed as the second argument to new RegExp(pattern, flags).

• Real-time match highlighting — Matches are highlighted directly in your test string as you type the pattern. Alternating colors (blue and green) distinguish adjacent matches, making it easy to see where one match ends and the next begins.
• Capture group inspection — Every capture group (numbered and named) is displayed for each match in the details panel. See exactly what each group captured, along with match position and length.
• Live replace preview — Enter a replacement string and see the result instantly. Supports all JavaScript replacement patterns: $1, $2 for numbered groups, $<name> for named groups, $& for the full match, and $` / $' for pre/post-match text.
• All JavaScript flags — Toggle any combination of the six JavaScript regex flags (g, i, m, s, u, y) with a single click. Active flags are visually highlighted so you always know which modifiers are in effect.
• Pattern validation — Invalid patterns are caught instantly with clear error messages. The input border turns red and the error message shows exactly what went wrong, helping you fix syntax issues quickly.
• Preset patterns — One-click presets for common regex patterns (email, URL, IP address, date, hex color, phone number) with realistic sample test strings. Great for learning and quick validation tasks.
• Quick reference — A collapsible reference panel with clickable tokens. Click any token to insert it into your pattern at the cursor position. Covers character classes, quantifiers, anchors, and groups.
• 100% client-side — All processing happens entirely in your browser. Your patterns and test data are never transmitted to any server. The tool works offline after the initial page load.
• No dependencies — Built with vanilla HTML, CSS, and JavaScript. No frameworks, no build tools, no third-party libraries. Loads instantly and runs on any modern browser.