Regular Expressions

// 1. Literal notation (preferred for static patterns) const regex1 = /pattern/flags; // 2. Constructor notation (for dynamic patterns) const regex2 = new RegExp('pattern', 'flags'); // Basic examples const simplePattern = /hello/; const caseInsensitive = /hello/i; const dynamicPattern = new RegExp(userName, 'i'); // Testing a pattern const text = 'Hello World'; console.log(/hello/i.test(text)); // true console.log(simplePattern.test(text)); // false (case-sensitive) // Matching a pattern const match = text.match(/hello/i); console.log(match); // ['Hello', index: 0, input: 'Hello World']

Character Classes: // Dot (.) - Matches any character except newline /h.t/ // Matches: hat, hot, hit, h9t, etc. // Character set [abc] - Matches any single character in the set /[aeiou]/ // Matches any vowel /[0-9]/ // Matches any digit /[a-z]/ // Matches any lowercase letter /[A-Z]/ // Matches any uppercase letter /[a-zA-Z]/ // Matches any letter // Negated set [^abc] - Matches any character NOT in the set /[^0-9]/ // Matches any non-digit /[^aeiou]/ // Matches any consonant // Shorthand character classes /\d/ // Digit [0-9] /\D/ // Non-digit [^0-9] /\w/ // Word character [a-zA-Z0-9_] /\W/ // Non-word character [^a-zA-Z0-9_] /\s/ // Whitespace (space, tab, newline) /\S/ // Non-whitespace Quantifiers: // * - Zero or more times /ab*c/ // Matches: ac, abc, abbc, abbbc // + - One or more times /ab+c/ // Matches: abc, abbc, abbbc (NOT ac) // ? - Zero or one time (optional) /colou?r/ // Matches: color, colour // {n} - Exactly n times /\d{3}/ // Matches exactly 3 digits: 123 // {n,} - n or more times /\d{3,}/ // Matches 3 or more digits: 123, 1234, 12345 // {n,m} - Between n and m times /\d{3,5}/ // Matches 3 to 5 digits: 123, 1234, 12345 // Practical examples const phonePattern = /\d{3}-\d{3}-\d{4}/; // 123-456-7890 const zipPattern = /\d{5}(-\d{4})?/; // 12345 or 12345-6789 const emailPattern = /\w+@\w+\.\w+/; // simple email pattern

// ^ - Start of string/line /^Hello/ // Matches 'Hello' only at the start /^\d{3}/ // Matches 3 digits at the start // $ - End of string/line /world$/ // Matches 'world' only at the end /\d{3}$/ // Matches 3 digits at the end // \b - Word boundary /\bcat\b/ // Matches 'cat' but not 'category' or 'concatenate' /\bJava\b/ // Matches 'Java' but not 'JavaScript' // \B - Non-word boundary /\Bcat/ // Matches 'scat' and 'concatenate' but not 'cat' // Examples const text1 = 'The cat sat on the mat'; console.log(/cat/.test(text1)); // true console.log(/^cat/.test(text1)); // false (cat not at start) console.log(/mat$/.test(text1)); // true (mat at end) console.log(/\bcat\b/.test(text1)); // true (cat is a word) console.log(/\bcat\b/.test('category')); // false (cat not a word) // Practical example: validate complete string const usernameRegex = /^[a-z0-9_]{3,16}$/; console.log(usernameRegex.test('john_doe')); // true console.log(usernameRegex.test('ab')); // false (too short) console.log(usernameRegex.test('John_Doe')); // false (has uppercase)

// Capturing groups ( ) const datePattern = /(\d{4})-(\d{2})-(\d{2})/; const dateString = '2024-01-15'; const match = dateString.match(datePattern); console.log(match[0]); // '2024-01-15' (full match) console.log(match[1]); // '2024' (first group - year) console.log(match[2]); // '01' (second group - month) console.log(match[3]); // '15' (third group - day) // Using groups with replace const formatted = dateString.replace(datePattern, '$2/$3/$1'); console.log(formatted); // '01/15/2024' // Named capturing groups const namedPattern = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/; const namedMatch = dateString.match(namedPattern); console.log(namedMatch.groups.year); // '2024' console.log(namedMatch.groups.month); // '01' console.log(namedMatch.groups.day); // '15' // Non-capturing groups (?: ) const nonCapturing = /(?:Mr|Ms|Mrs)\.\s(\w+)/; const name = 'Mr. Smith'; const nameMatch = name.match(nonCapturing); console.log(nameMatch[0]); // 'Mr. Smith' console.log(nameMatch[1]); // 'Smith' (only this is captured) // Alternation with groups const filePattern = /\.(jpg|jpeg|png|gif)$/i; console.log(filePattern.test('photo.jpg')); // true console.log(filePattern.test('image.PNG')); // true console.log(filePattern.test('doc.pdf')); // false // Backreferences - reference earlier captures const duplicatePattern = /(\w+)\s\1/; // Matches repeated words console.log(duplicatePattern.test('hello hello')); // true console.log(duplicatePattern.test('hello world')); // false // Find repeated words in text const text = 'This this is a test test.'; const duplicates = text.match(/(\w+)\s\1/g); console.log(duplicates); // ['This this', 'test test']

// Positive lookahead (?= ) // Matches if followed by pattern const pricePattern = /\d+(?= dollars)/; 'Price: 50 dollars'.match(pricePattern); // ['50'] 'Price: 50 euros'.match(pricePattern); // null // Negative lookahead (?! ) // Matches if NOT followed by pattern const notDollarPattern = /\d+(?! dollars)/; '50 euros'.match(notDollarPattern); // ['50'] '50 dollars'.match(notDollarPattern); // null (50 is followed by ' dollars') // Positive lookbehind (?<= ) // Matches if preceded by pattern const afterDollarPattern = /(?<=\$)\d+/; 'Price: $50'.match(afterDollarPattern); // ['50'] 'Price: 50'.match(afterDollarPattern); // null // Negative lookbehind (?<! ) // Matches if NOT preceded by pattern const notAfterDollarPattern = /(?<!\$)\d+/; 'Price: 50'.match(notAfterDollarPattern); // ['50'] 'Price: $50'.match(notAfterDollarPattern); // null // Practical example: Password validation // At least 8 chars, must have uppercase, lowercase, and digit const passwordPattern = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/; console.log(passwordPattern.test('Password123')); // true console.log(passwordPattern.test('password123')); // false (no uppercase) console.log(passwordPattern.test('PASSWORD123')); // false (no lowercase) console.log(passwordPattern.test('Password')); // false (no digit) console.log(passwordPattern.test('Pass123')); // false (too short)

// g - Global: find all matches (not just first) const textWithNumbers = 'I have 2 cats and 3 dogs'; console.log(textWithNumbers.match(/\d/)); // ['2'] (first match only) console.log(textWithNumbers.match(/\d/g)); // ['2', '3'] (all matches) // i - Case insensitive console.log(/hello/i.test('HELLO')); // true console.log(/hello/.test('HELLO')); // false // m - Multiline: ^ and $ match line boundaries const multilineText = `Line 1 Line 2 Line 3`; console.log(multilineText.match(/^Line/g)); // ['Line'] (only first) console.log(multilineText.match(/^Line/gm)); // ['Line', 'Line', 'Line'] // s - Dotall: . matches newlines const textWithNewline = 'Hello\nWorld'; console.log(/Hello.World/.test(textWithNewline)); // false console.log(/Hello.World/s.test(textWithNewline)); // true // u - Unicode: enable Unicode support const emoji = '😀'; console.log(/^.$/.test(emoji)); // false (emoji is 2 code units) console.log(/^.$/u.test(emoji)); // true (with Unicode support) // y - Sticky: match from lastIndex position const stickyRegex = /\d+/y; const numbers = '123 456 789'; stickyRegex.lastIndex = 0; console.log(stickyRegex.exec(numbers)); // ['123'] stickyRegex.lastIndex = 4; console.log(stickyRegex.exec(numbers)); // ['456'] // Combining flags const pattern = /hello/gi; // Global and case-insensitive const text = 'Hello hello HELLO'; console.log(text.match(pattern)); // ['Hello', 'hello', 'HELLO']

// Email validation (basic) const emailRegex = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/; console.log(emailRegex.test('user@example.com')); // true // Phone number (US format) const phoneRegex = /^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$/; console.log(phoneRegex.test('(123) 456-7890')); // true console.log(phoneRegex.test('123-456-7890')); // true console.log(phoneRegex.test('1234567890')); // true // URL validation const urlRegex = /^https?:\/\/[\w\-]+(\.[\w\-]+)+[/#?]?.*$/; console.log(urlRegex.test('https://example.com')); // true console.log(urlRegex.test('http://sub.example.com/path')); // true // Credit card number (basic check) const cardRegex = /^\d{4}[\s-]?\d{4}[\s-]?\d{4}[\s-]?\d{4}$/; console.log(cardRegex.test('1234-5678-9012-3456')); // true // Hex color code const hexRegex = /^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/; console.log(hexRegex.test('#FF5733')); // true console.log(hexRegex.test('#F57')); // true // IPv4 address const ipRegex = /^(\d{1,3}\.){3}\d{1,3}$/; console.log(ipRegex.test('192.168.1.1')); // true // Date (YYYY-MM-DD) const dateRegex = /^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/; console.log(dateRegex.test('2024-01-15')); // true // Strong password // Min 8 chars, uppercase, lowercase, digit, special char const strongPassRegex = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/; console.log(strongPassRegex.test('Secure@123')); // true // Username (alphanumeric, underscore, 3-16 chars) const usernameRegex = /^[a-zA-Z0-9_]{3,16}$/; console.log(usernameRegex.test('john_doe123')); // true // HTML tag const tagRegex = /<([a-z]+)([^<]+)*(?:>(.*)<\/\1>|\s+\/>)/; console.log(tagRegex.test('<div class="test">content</div>')); // true

// 1. Extract all URLs from text function extractUrls(text) { const urlPattern = /https?:\/\/[\w\-]+(\.[\w\-]+)+[/#?]?[^\s]*/gi; return text.match(urlPattern) || []; } const text = 'Visit https://example.com or http://test.org for info'; console.log(extractUrls(text)); // ['https://example.com', 'http://test.org'] // 2. Format phone numbers function formatPhone(phone) { const cleaned = phone.replace(/\D/g, ''); // Remove non-digits const match = cleaned.match(/^(\d{3})(\d{3})(\d{4})$/); if (match) { return `(${match[1]}) ${match[2]}-${match[3]}`; } return phone; } console.log(formatPhone('1234567890')); // (123) 456-7890 console.log(formatPhone('123-456-7890')); // (123) 456-7890 // 3. Sanitize input (remove special characters) function sanitizeInput(input) { return input.replace(/[^a-zA-Z0-9\s]/g, ''); } console.log(sanitizeInput('Hello<script>alert("XSS")</script>World')); // HelloscriptalertXSSscriptWorld // 4. Validate credit card with Luhn algorithm check function validateCard(cardNumber) { const cleaned = cardNumber.replace(/\s/g, ''); // Check format if (!/^\d{13,19}$/.test(cleaned)) { return false; } // Luhn algorithm let sum = 0; let isEven = false; for (let i = cleaned.length - 1; i >= 0; i--) { let digit = parseInt(cleaned[i]); if (isEven) { digit *= 2; if (digit > 9) digit -= 9; } sum += digit; isEven = !isEven; } return sum % 10 === 0; } // 5. Highlight search terms function highlightText(text, searchTerm) { const regex = new RegExp(`(${searchTerm})`, 'gi'); return text.replace(regex, '<mark>$1</mark>'); } console.log(highlightText('JavaScript is great', 'javascript')); // <mark>JavaScript</mark> is great // 6. Parse CSV line function parseCSV(line) { const pattern = /("(?:[^"]|"")*"|[^,]+)(?:,|$)/g; const fields = []; let match; while ((match = pattern.exec(line)) !== null) { fields.push(match[1].replace(/^"|"$/g, '').replace(/""/g, '"')); } return fields; } console.log(parseCSV('John,Doe,"123 Main St, Apt 4",Boston')); // ['John', 'Doe', '123 Main St, Apt 4', 'Boston'] // 7. Extract hashtags function extractHashtags(text) { return text.match(/#[a-zA-Z0-9_]+/g) || []; } console.log(extractHashtags('Love #JavaScript and #WebDev!')); // ['#JavaScript', '#WebDev']