1. Open the RegEx Tester
2. RegEx Pattern: Enter your regular expression (e.g., `/\d{3}-\d{4}/`)
3. Flags: Select options:
   • g (global): Find all occurrences
   • i (case-insensitive): Ignore case
   • m (multiline): `^` and `$` match line start/end
   • s (dotAll): `.` matches newlines too
   • u (unicode): Full Unicode support
4. Test String: Enter the text to test
5. See real-time results with match highlighting

Advanced Features

• Match Groups: View capture groups `(...)` extracted
• Replace Mode: Test replacements with `$1`, `$2` for groups
• Pattern Library: Predefined templates for common cases (email, URL, phone, SSN)
• Debug Mode: Visualize step-by-step how the regex matches text
• Performance: Warning if regex is inefficient (ReDoS risk)

Practical Example: Validate Email

// 1. Enter pattern
/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/

// 2. Flags: none (case-sensitive match)

// 3. Test string
john.doe@example.com
JANE@EXAMPLE.COM
invalid-email
test@subdomain.example.co.uk

// 4. Results
✅ john.doe@example.com (full match)
✅ JANE@EXAMPLE.COM (full match)
❌ invalid-email (no match)
✅ test@subdomain.example.co.uk (full match)

Code Examples

JavaScript

// Test regex
const regex = /^\d{3}-\d{3}-\d{4}$/
regex.test('555-123-4567') // true
regex.test('555.123.4567') // false

// Extraction with match()
const text = "Email: test@example.com, info@test.com"
const emails = text.match(/[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}/gi)
console.log(emails) // ["test@example.com", "info@test.com"]

// Replace with capture groups
const date = "12/25/2024"
const formatted = date.replace(/(\d{2})\/(\d{2})\/(\d{4})/, '$3-$1-$2')
console.log(formatted) // "2024-12-25"

// Form validation in React
function validateEmail(email) {
  const regex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/
  return regex.test(email)
}

// Extract groups with exec()
const regex = /(\d{2})\/(\d{2})\/(\d{4})/
const match = regex.exec("Date: 12/25/2024")
if (match) {
  console.log(`Month: ${match[1]}, Day: ${match[2]}, Year: ${match[3]}`)
}

Python

import re

# Test regex
pattern = r'^\d{3}-\d{3}-\d{4}$'
re.match(pattern, '555-123-4567')  # Match object
re.match(pattern, '555.123.4567')  # None

# Extraction
text = "Email: test@example.com, info@test.com"
emails = re.findall(r'[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}', text, re.IGNORECASE)
print(emails)  # ['test@example.com', 'info@test.com']

# Replace with groups
date = "12/25/2024"
formatted = re.sub(r'(\d{2})/(\d{2})/(\d{4})', r'\3-\1-\2', date)
print(formatted)  # "2024-12-25"

# SSN validation
def validate_ssn(ssn):
    pattern = r'^\d{3}-\d{2}-\d{4}$'
    return bool(re.match(pattern, ssn))

# Log parsing with named groups
log = '192.168.1.1 - - [10/Dec/2024:13:55:36] "GET /api/users HTTP/1.1" 200'
pattern = r'(?P\S+).*\[(?P[\w:/]+).*\] "(?P\w+) (?P\S+).*" (?P\d{3})'
match = re.search(pattern, log)
print(match.groupdict())
# {'ip': '192.168.1.1', 'date': '10/Dec/2024:13:55:36', 'method': 'GET', 'path': '/api/users', 'status': '200'}

PHP

<?php
// Test regex
$pattern = '/^\d{3}-\d{3}-\d{4}$/';
preg_match($pattern, '555-123-4567');  // 1 (true)
preg_match($pattern, '555.123.4567');  // 0 (false)

// Extraction
$text = "Email: test@example.com, info@test.com";
preg_match_all('/[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}/i', $text, $matches);
print_r($matches[0]);  // Array(['test@example.com', 'info@test.com'])

// Replace
$date = "12/25/2024";
$formatted = preg_replace('/(\d{2})\/(\d{2})\/(\d{4})/', '$3-$1-$2', $date);
echo $formatted;  // "2024-12-25"

// SSN validation
function validateSSN($ssn) {
    return preg_match('/^\d{3}-\d{2}-\d{4}$/', $ssn);
}

// Input sanitization (remove non-alphanumeric)
$input = "Hello, World! 123";
$clean = preg_replace('/[^a-zA-Z0-9\s]/', '', $input);
echo $clean;  // "Hello World 123"
?>

Common RegEx Patterns (US/International)

RegEx Tester Comparison

When to Use THEJORD RegEx Tester

• ✅ Privacy-first: No data sent to external servers (all local processing)
• ✅ Multi-region patterns: Library with regex for US, EU, international formats
• ✅ Fast: No loading, instant processing
• ✅ Free: No usage limits or registration required
• ✅ Multi-language: Supports JavaScript, Python, PHP, Java syntax

Best Practices and Performance

⚠️ Avoid Catastrophic Backtracking (ReDoS)

Some regex can cause exponential execution time on specific input:

// ❌ DANGEROUS - ReDoS vulnerability
/(a+)+b/
/(\w+\s?)*$/

// ✅ SAFE - Non-nested quantifiers
/a+b/
/[\w\s]+$/

Rule: Avoid nested quantifiers `(a+)+`, `(a*)*`, `(a+)*`.

✅ Optimize Regex

• Use anchors: `^` and `$` limit matching scope
• Specific classes: `\d` instead of `[0-9]`, `\w` instead of `[a-zA-Z0-9_]`
• Non-capture groups: `(?:...)` instead of `(...)` if extraction not needed
• Efficient alternation: `(jpg|png|gif)` instead of `(jpg)|(png)|(gif)`

🧪 Test Edge Cases

Always verify with boundary input:

• Empty string: `""`
• Single character: `"a"`
• Special characters: `"!@#$%"`
• Unicode/emoji: `"hello 👋"`
• Very long: 10,000+ characters
• Newlines and tabs: `"hello\nworld\t"`

Frequently Asked Questions (FAQ)

What's the difference between /g and without g flag?

Without `g`, regex stops at the first match. With `g` (global), it finds all occurrences in the text.

const text = "cat bat rat"
text.match(/at/)   // ["at"] (only first match)
text.match(/at/g)  // ["at", "at", "at"] (all matches)

How do I match a special character like . or *?

Use backslash `\` to escape metacharacters:

/\./ → matches literal dot "."
/\*/ → matches asterisk "*"
/\+/ → matches plus "+"
/\(/ → matches parenthesis "("

Regex for strong password validation?

// Min 8 chars, at least: 1 uppercase, 1 lowercase, 1 number, 1 special
/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/

Explanation: Uses positive lookahead `(?=...)` to verify each condition without consuming characters.

How to extract content between HTML tags?

⚠️ Warning: Regex is NOT suitable for complex HTML parsing. For HTML use a parser (DOMParser, Cheerio, BeautifulSoup).

For simple cases:

// Extract text in 

/
(.*?)<\/p>/gs

// Extract href from 
/

Input: "
hello
world
"

/
.*<\/p>/  → match: "
hello
world
" (entire)
/
.*?<\/p>/ → match: "
hello
" (only first tag)

/hello/.test("HELLO")   // false
/hello/i.test("HELLO")  // true