What is a regular expression?

A regular expression (regex) is a pattern-matching language for searching and manipulating text. Regex uses special characters to define search patterns: "\d" matches digits, "." matches any character, "*" means zero or more repetitions. Example: "\d{3}-\d{4}" matches phone numbers like "555-1234". Regex is used in text editors, programming languages, data validation, log parsing, and search-replace operations across virtually all modern development tools.

What do regex flags (g, i, m) do?

Flags modify regex behavior. "g" (global) finds all matches instead of stopping at the first match. "i" (case-insensitive) makes "ABC" match "abc". "m" (multiline) makes "^" and "$" match line boundaries instead of string boundaries. "s" (dotall) makes "." match newlines. "u" (unicode) enables Unicode property escapes. Example: /hello/gi finds all occurrences of "hello", "Hello", "HELLO" in text.

How do I match email addresses with regex?

Basic email pattern: "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$". This matches "user@example.com". However, full RFC 5322 email validation is extremely complex (thousands of characters). For production: use basic patterns for UI validation, then verify via email confirmation link. Never rely solely on regex for email validation; addresses like "user+tag@domain.co.uk" have many edge cases.

What are capture groups in regex?

Capture groups extract parts of matches using parentheses. Pattern "(\d{3})-(\d{4})" applied to "555-1234" captures two groups: group 1 = "555", group 2 = "1234". Non-capturing groups "(?:...)" match but do not capture. Named groups "(? ...)" assign names to captures. Use captures for: extracting data (phone area codes), search-replace ($1, $2 placeholders), or conditional matching.

Why does my regex match too much or too little?

Common issues: greedy vs lazy quantifiers. ".*" (greedy) matches as much as possible; ".*?" (lazy) matches as little as possible. Example: in " hello world ", pattern " .* " matches entire string (greedy); " .*? " matches each tag separately (lazy). Anchors matter: "abc" matches "abc" anywhere; "^abc$" requires entire string to be exactly "abc". Test edge cases and boundary conditions.

Can regex validate all formats (URLs, phone numbers, IBANs)?

Regex works for simple formats but fails for complex ones. URLs: regex cannot validate all valid URLs per RFC 3986 (use URL parser libraries). Phone numbers: regex handles formatting (123-456-7890) but not validity (real phone numbers require carrier databases). IBANs: regex checks format but not MOD-97 checksum (use specialized validators). Use regex for format checking; use dedicated libraries/APIs for comprehensive validation.

Regex Tester - Test Regular Expressions Online

How to Test Regular Expressions

Enter your regex pattern (e.g., \d3-\d4)
Add test string to match against
Choose flags (g=global, i=case-insensitive, m=multiline)
View matches with highlighting and capture groups

Regex Tester Features

Pattern Validation: Checks if regex syntax is valid
Match Highlighting: Visual highlighting of matched text
Group Capture Display: Shows captured groups separately
Common Pattern Library: Pre-built patterns for email, URL, phone, etc.
Multi-flag Support: g (global), i (case-insensitive), m (multiline)
Real-Time Testing: Matches update as you type

What are Regular Expressions?

Regular expressions (regex) are patterns for matching text. They're used to validate input, extract data, and search-and-replace operations. A regex like ^\d3-\d4$ matches phone numbers in format 555-1234.

Common patterns: \d matches digits, \w matches word characters, . matches any character, * means zero or more, + means one or more. Flags modify behavior: g finds all matches (not just first), i ignores case, m makes ^ and $ match line boundaries.

Use cases: Validating email addresses and phone numbers, extracting URLs from text, parsing log files, find-and-replace in code editors, sanitizing user input, and web scraping. Warning: Complex regex can be slow on large text—test performance before using in production.

Privacy & Security

All regex testing happens locally in your browser. Your patterns and test strings are never uploaded to servers, stored, or logged. The testing uses JavaScript, keeping your data completely private.