Regex Tester

A regular expression is a sequence of characters that defines a search pattern. Regex is used for finding, matching, and manipulating text based on patterns rather than exact strings. It's incredibly powerful for tasks like validating email addresses, extracting data from text, find-and-replace operations, and parsing log files.

Enter your regex pattern in the pattern field and paste your test text in the text area. The tool will highlight all matches in real-time, showing you exactly what your pattern captures. If nothing highlights, your pattern isn't matching. Use this visual feedback to refine your pattern until it matches exactly what you need.

The most common flags are: 'g' (global) finds all matches instead of just the first one, 'i' (case-insensitive) ignores uppercase/lowercase differences, 'm' (multiline) makes ^ and $ match line starts/ends instead of just string start/end, and 's' (dotall) makes . match newline characters. Combine flags like 'gi' for global case-insensitive matching.

For email: try /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/ for basic validation. For URLs: /https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*)/ works for most cases. Note that perfect email/URL validation is extremely complex - these patterns handle most common cases.

The dot (.) matches any character. The asterisk (*) means 'zero or more times' while plus (+) means 'one or more times'. So .* matches zero or more of any character (including matching nothing), while .+ requires at least one character to match. For example, 'a.*b' matches 'ab' or 'a123b', but 'a.+b' only matches 'a123b'.

By default, quantifiers like *, +, and {n,m} are greedy - they match as much as possible. Add a question mark (?) after the quantifier to make it non-greedy (lazy), matching as little as possible. For example, in the text '<div>hello</div><div>world</div>', /<div>.*<\/div>/ matches the entire string (greedy), while /<div>.*?<\/div>/ matches each tag separately (non-greedy).

Capturing groups use parentheses () to extract specific parts of a match. For example, /user-(\d+)/ captures the numeric user ID. The captured value can be accessed in most languages using match groups. Use (?:...) for non-capturing groups when you need grouping for quantifiers but don't want to capture the value.

Paste sample log entries into the test string area and create patterns to extract timestamps, IP addresses, status codes, or error messages. Visual highlighting shows exactly what matches, helping you refine patterns before deploying them in log analysis tools like Logstash, Splunk, or custom scripts.

Lookaheads (?=...) and lookbehinds (?<=...) assert that a pattern exists ahead or behind without including it in the match. For example, /\d+(?= dollars)/ matches numbers followed by ' dollars' but only captures the number. Negative lookaheads (?!...) and negative lookbehinds (?<!...) assert the pattern does NOT exist.