Dev Tools

Regex Tester

Test regular expressions with real-time match highlighting, group extraction, replace mode, and a library of common patterns.

Quick Answer

A regular expression (regex) is a pattern used to match character combinations in strings. Common flags include g (global — match all occurrences), i (case-insensitive), and m (multiline). Key tokens: \d matches digits, \w matches word characters, . matches any character, and quantifiers like + (one or more) and * (zero or more) control repetition.

//g

About This Tool

The Regex Tester is a free online tool for building, testing, and debugging regular expressions in real time. As you type your pattern and test string, matches are highlighted instantly with color-coded markers, and capturing groups are extracted into a structured table. No server round-trips — everything runs in your browser using JavaScript’s native RegExp engine.

Regular expressions are one of the most powerful tools in a developer’s toolkit. They are used for input validation (email, phone, dates), search-and-replace operations, log file analysis, data extraction, and URL routing. However, regex syntax can be cryptic. This tool’s Explain feature breaks your pattern into individual tokens and describes each one in plain English, making it easier to understand and debug complex expressions.

The built-in pattern library includes 10 commonly needed regular expressions for emails, URLs, phone numbers, dates, IP addresses, hex colors, HTML tags, credit cards, postal codes, and strong password validation. Click any pattern to load it instantly. In Replace mode, you can test find-and-replace operations using backreferences ($1, $2) for capturing groups.

Performance tip: avoid catastrophic backtracking by being specific with your quantifiers. Patterns like (a+)+ or (a|a)* can cause exponential time complexity. This tool caps matches at 1,000 to prevent browser freezes. All data stays in your browser — nothing is ever sent to a server.

Frequently Asked Questions

What is a regular expression (regex)?
A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. It is used for pattern matching within strings -- for example, finding all email addresses in a document, validating input formats, or performing find-and-replace operations. Regex is supported by virtually every programming language and many text editors.
What do the regex flags (g, i, m, s) mean?
The 'g' (global) flag finds all matches, not just the first one. The 'i' (case-insensitive) flag ignores upper/lowercase differences. The 'm' (multiline) flag makes ^ and $ match the start/end of each line, not just the whole string. The 's' (dotAll) flag makes the . character match newlines as well. You can combine multiple flags.
How do I match special characters literally?
Characters like . * + ? ^ $ { } [ ] ( ) | \ have special meaning in regex. To match them literally, precede them with a backslash. For example, \. matches a literal period, \( matches a literal opening parenthesis, and \\ matches a literal backslash.
What are capturing groups and how do I use them?
Parentheses () create capturing groups that extract specific parts of a match. For example, the pattern (\d{3})-(\d{4}) matching '555-1234' would capture '555' as group 1 and '1234' as group 2. You can reference these groups in replacement strings with $1, $2, etc. Non-capturing groups (?:...) group without capturing.
Is my data stored or sent to a server?
No. All regex matching, replacing, and analysis happens entirely in your browser using JavaScript's built-in RegExp engine. Your patterns and test strings never leave your device -- nothing is sent to any server, stored, or logged. The tool works completely offline once the page loads.

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