🔣 Regex Tester

Test regular expression patterns in real-time and view match results. Provides flag settings, group capture, and common pattern library.

Regex Pattern

Flags

g (global) i (case insensitive) m (multiline) s (dotAll) u (unicode) y (sticky)

Test String

Common Patterns

GUIDE

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1. Regular Expression Basics

Regular expressions (Regex) are powerful tools for defining, searching, extracting, and replacing string patterns. Match literally (\d for digits, \w for word characters) or use special characters for complex patterns. Example: ^[a-z]+$ means strings containing only lowercase letters. ^ is start, $ is end, [a-z] is a to z, + means one or more. Supported in almost all programming languages and text editors, essential for input validation, data parsing, and text replacement.

2. Common Pattern Collection

Frequently used regex patterns in practice: Email: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$, Phone (US): ^(\+1)?[-.\s]?$?[0-9]{3}$?[-.\s]?[0-9]{3}[-.\s]?[0-9]{4}$, URL: ^https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b, Date (YYYY-MM-DD): ^\d{4}-\d{2}-\d{2}$, Password (min 8 chars, upper+lower+digit+special): ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$. Use these patterns as a base and modify for your project.

3. Regex Flags Explained

Regex flags control search behavior. g(global): finds all matches, not just the first. i(ignoreCase): case-insensitive matching (/hello/i matches Hello, HELLO). m(multiline): ^ and $ match start/end of each line. s(dotAll): . matches newline characters (\n). u(unicode): properly handles full Unicode (emojis, etc.). y(sticky): starts search only at lastIndex position. Example: /hello/gi finds all hello ignoring case.

4. Capture Groups and Backreferences

Parentheses () create groups that capture matched portions for later use. Example: /(\d{4})-(\d{2})-(\d{2})/ where $1 is year, $2 is month, $3 is day. JavaScript's match() result array includes capture groups. Backreferences \1, \2 match the same text as previous groups. Example: /(\w+)\s+\1/ matches "hello hello". Non-capturing groups (?:) group without capturing for better performance.

5. Lookahead and Lookbehind

Lookahead and lookbehind check for patterns before/after without including them in the match. Positive lookahead (?=): /\d(?=px)/ matches digits before px ("100" in "100px"). Negative lookahead (?!): /\d(?!px)/ matches digits not before px. Positive lookbehind (?<=): /(?<=\$)\d+/ matches digits after $ ("100" in "$100"). Negative lookbehind (?<!): the opposite. Frequently used in password validation to check for specific character inclusion.

6. Regex Performance Optimization

Complex regex can cause performance issues. Be as specific as possible ([a-z]+ rather than .*). Replace unnecessary capturing groups with non-capturing groups (?:). Use lazy quantifiers (*?, +?) instead of greedy (*,+) to reduce backtracking. For large texts, string methods (indexOf, startsWith) may be faster than regex. Pre-compile regex for reuse (don't repeatedly create new RegExp). Beware of ReDoS (Regular Expression Denial of Service) attacks - never use user input directly as regex patterns.