Tester Regex

// Match all email addresses in a string
const text = 'Contact us at support@example.com or sales@example.com'
const emailRegex = /[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/g

const matches = text.matchAll(emailRegex)
for (const match of matches) {
  console.log(match[0], 'at index', match.index)
}
// → "support@example.com" at index 14
// → "sales@example.com" at index 37

// Named capture groups (ES2018+)
const dateRegex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/
const result = '2026-03-30'.match(dateRegex)
console.log(result.groups)
// → { year: "2026", month: "03", day: "30" }

// Replace with a callback
'hello world'.replace(/\b\w/g, c => c.toUpperCase())
// → "Hello World"

import re

# Find all IPv4 addresses
text = 'Server 192.168.1.1 responded, fallback to 10.0.0.255'
pattern = r'\b(?:\d{1,3}\.){3}\d{1,3}\b'

matches = re.findall(pattern, text)
print(matches)  # → ['192.168.1.1', '10.0.0.255']

# Named groups and match objects
date_pattern = r'(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})'
m = re.search(date_pattern, 'Released on 2026-03-30')
if m:
    print(m.group('year'))   # → '2026'
    print(m.group('month'))  # → '03'

# Compile for repeated use (faster in loops)
compiled = re.compile(r'\b[A-Z][a-z]+\b')
words = compiled.findall('Hello World Foo bar')
print(words)  # → ['Hello', 'World', 'Foo']

package main

import (
	"fmt"
	"regexp"
)

func main() {
	// Find all matches
	re := regexp.MustCompile(`\b[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}\b`)
	text := "Contact support@example.com or sales@example.com"
	matches := re.FindAllString(text, -1)
	fmt.Println(matches)
	// → [support@example.com sales@example.com]

	// Named capture groups
	dateRe := regexp.MustCompile(`(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})`)
	match := dateRe.FindStringSubmatch("2026-03-30")
	for i, name := range dateRe.SubexpNames() {
		if name != "" {
			fmt.Printf("%s: %s\n", name, match[i])
		}
	}
	// → year: 2026
	// → month: 03
	// → day: 30

	// Replace with a function
	result := re.ReplaceAllStringFunc(text, func(s string) string {
		return "[REDACTED]"
	})
	fmt.Println(result)
	// → Contact [REDACTED] or [REDACTED]
}

# Find lines matching an IP address pattern
grep -E '\b([0-9]{1,3}\.){3}[0-9]{1,3}\b' access.log

# Extract email addresses from a file
grep -oE '[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}' contacts.txt

# Replace dates from YYYY-MM-DD to DD/MM/YYYY using sed
echo "2026-03-30" | sed -E 's/([0-9]{4})-([0-9]{2})-([0-9]{2})/\3\/\2\/\1/'
# → 30/03/2026

# Count matches per file in a directory
grep -rcE 'TODO|FIXME|HACK' src/
# → src/main.js:3
# → src/utils.js:1