Regular Expression Testing Guide

Regular expressions (regex) are powerful patterns for matching text. Whether you're validating input, searching logs, or parsing data, regex is an essential tool for developers, data analysts, and power users. This tool helps you test and visualize regex matches in real-time.

Common Regex Patterns

Practical Regex Examples

Email Address

\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b

Matches: user@example.com, john.doe+filter@domain.co.uk

Phone Numbers

\d{3}-\d{3}-\d{4}

Matches: 123-456-7890

URL

https?://[^\s]+

Matches: http://example.com, https://site.com/path

IPv4 Address

\b(?:\d{1,3}\.){3}\d{1,3}\b

Matches: 192.168.1.1, 10.0.0.1

Hexadecimal Color

#[0-9A-Fa-f]{6}\b

Matches: #FF5733, #000000

Date (YYYY-MM-DD)

\d{4}-\d{2}-\d{2}

Matches: 2025-01-15, 2024-12-31

HTML Tag

<([a-z]+)([^<>]*)>.*?</\1>

Matches: <div>content</div>, <p class="text">text</p>

Regex Flags Explained

Flag	Name	Description
i	Case Insensitive	Makes pattern match both uppercase and lowercase. /hello/i matches "Hello", "HELLO"
m	Multiline	Makes ^ and $ match line beginnings/ends, not just string start/end
s	Dotall	Makes . match newlines too (normally it doesn't)

Capturing Groups

Parentheses create capturing groups that extract specific parts of matches:

(\d{3})-(\d{3})-(\d{4})

Applied to "123-456-7890":

• Group 1: 123
• Group 2: 456
• Group 3: 7890

Non-Capturing Groups

Use (?:...) to group without capturing:

(?:https?|ftp)://[^\s]+

Groups for alternation but doesn't capture "http" or "https"

Common Regex Pitfalls

1. Greedy vs Lazy: .* is greedy (matches as much as possible), .*? is lazy (matches as little as possible)
2. Escaping Special Characters: To match literal ., *, +, etc., escape them: \., \*
3. Performance: Complex patterns with many alternations or backtracking can be slow
4. Unicode: Basic regex doesn't handle all Unicode characters. Consider using Unicode-aware patterns when needed

Testing Best Practices

• Start Simple: Build complex patterns incrementally, testing each addition
• Test Edge Cases: Try empty strings, special characters, very long strings
• Use Online Tools: Test your regex in multiple tools to ensure compatibility
• Document Complex Patterns: Add comments explaining what each part does
• Consider Alternatives: Sometimes string methods are simpler than regex

Real-World Applications

Log File Analysis

Extract error messages, timestamps, or specific events from logs using regex patterns.

Data Validation

Validate user input like emails, phone numbers, credit cards, postal codes, etc.

Text Processing

Find and replace patterns in documents, code refactoring, bulk text transformations.

Web Scraping

Extract specific data from HTML or text responses when parsing is overkill.

Search and Replace in IDEs

Most code editors support regex in find/replace, enabling powerful refactoring.