Free Online Regex Tester & Validator 2026 � Test Regular Expressions Instantly

Welcome to Code Formatter's Regex Tester � the ultimate tool for testing, validating, and debugging regular expressions in real-time. Whether you're a seasoned developer building complex pattern matching systems or a beginner learning the fundamentals of regex, our free online regex tester provides instant visual feedback with highlighted matches, detailed match information, and a comprehensive library of common patterns.

Regular expressions are incredibly powerful but notoriously difficult to master. Syntax errors are easy to make, and patterns often behave unexpectedly. Our regex tester eliminates the guesswork by showing you exactly what your pattern matches (and what it doesn't) as you type. All processing happens 100% client-side in your browser, ensuring your sensitive data and proprietary patterns remain completely private.

What Are Regular Expressions?

Regular expressions (regex or regexp) are sequences of characters that define search patterns. They're a fundamental tool in programming, used for:

• Text Validation: Checking if user input matches expected formats (emails, phone numbers, passwords)
• Search & Replace: Finding and modifying specific text patterns in documents or code
• Data Extraction: Pulling structured information from unstructured text (scraping, log parsing)
• Input Sanitization: Removing or escaping dangerous characters to prevent security vulnerabilities
• Syntax Highlighting: Identifying keywords, strings, and comments in code editors

How to Use Our Regex Tester

1. Enter Your Pattern: Type your regular expression in the pattern input field. The pattern is automatically wrapped with forward slashes.
2. Select Flags: Click the flag buttons (g, i, m, s, u) to enable or disable regex modifiers. Common combinations include gi for global case-insensitive matching.
3. Add Test String: Enter or paste the text you want to test against your pattern. Matches are highlighted instantly as you type.
4. Review Matches: The results panel shows all matches with their positions and captured groups. Use this information to refine your pattern.
5. Use Common Patterns: Click any pattern chip to instantly load pre-built patterns for emails, URLs, phone numbers, and more.

Understanding Regex Flags

Regex flags modify how the pattern matching engine behaves. Our tester supports all JavaScript regex flags:

g - Global Flag

Matches all occurrences of the pattern in the string, not just the first one. Without this flag, only the first match is returned. Essential for find-all operations.

i - Case Insensitive Flag

Makes the pattern match both uppercase and lowercase letters. /hello/i matches "hello", "HELLO", "HeLLo", etc.

m - Multiline Flag

Changes the behavior of ^ and $ anchors. With this flag, they match the start and end of each line, not just the entire string.

s - DotAll Flag

Makes the dot . metacharacter match newline characters as well. By default, dot matches any character except newlines.

u - Unicode Flag

Enables full Unicode matching, allowing patterns to correctly handle characters outside the Basic Multilingual Plane (emojis, mathematical symbols, etc.).

Essential Regex Syntax Reference

• . � Matches any single character (except newline without s flag)
• * � Matches zero or more of the preceding element
• + � Matches one or more of the preceding element
• ? � Matches zero or one of the preceding element (optional)
• ^ � Matches the start of the string (or line with m flag)
• $ � Matches the end of the string (or line with m flag)
• [abc] � Matches any single character in the brackets (character class)
• [^abc] � Matches any character NOT in the brackets
• \d � Matches any digit (equivalent to [0-9])
• \w � Matches any word character (letters, digits, underscore)
• \s � Matches any whitespace character (space, tab, newline)
• (group) � Creates a capturing group for extraction
• (?:group) � Creates a non-capturing group
• a|b � Matches either "a" or "b" (alternation)
• {n,m} � Matches between n and m occurrences (quantifier)

Common Regex Patterns

Email Validation

[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

Validates standard email addresses. Note that perfect email validation via regex is notoriously complex; this pattern covers most common formats.

URL Matching

https?:\/\/[^\s]+

Matches HTTP and HTTPS URLs, including paths, query strings, and fragments.

Phone Number

\+?[1-9]\d{1,14}

Matches international phone numbers in E.164 format (with optional plus sign).

IPv4 Address

\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

Matches IPv4 addresses. For strict validation, you'd need additional logic to ensure each octet is 0-255.

Best Practices for Writing Regex

• Start Simple: Build your pattern incrementally, testing after each addition
• Be Specific: Avoid overly broad patterns that match unintended text
• Use Non-Capturing Groups: Use (?:...) when you don't need the captured value
• Escape Special Characters: Remember to escape . * + ? ^ $ { } [ ] \ | ( )
• Consider Edge Cases: Test with empty strings, very long strings, and special characters
• Document Complex Patterns: Add comments explaining what each part of the regex does
• Avoid Catastrophic Backtracking: Be careful with nested quantifiers that can cause exponential matching time

Frequently Asked Questions

Code Formatter � 2026. Professional developer tools built with privacy and performance in mind.