Regular Expressions

// Creating regular expressions
const regex1 = /pattern/;              // Literal notation
const regex2 = new RegExp("pattern");  // Constructor

// Flags
const caseInsensitive = /pattern/i;    // Case insensitive
const globalSearch = /pattern/g;       // Find all matches
const multiLine = /pattern/m;          // Multi-line mode
const combined = /pattern/gim;         // Multiple flags

// Basic testing
const pattern = /hello/i;
console.log(pattern.test("Hello World")); // true
console.log(pattern.test("Hi there"));    // false

// Finding matches
const text = "Hello World, hello universe";
console.log(text.match(/hello/));   // ["hello", index: 13...]
console.log(text.match(/hello/g));  // ["hello"]
console.log(text.match(/hello/gi)); // ["Hello", "hello"]

// exec() - more detail
const regex = /hello/gi;
let match;
while ((match = regex.exec(text)) !== null) {
    console.log(`Found "${match[0]}" at index ${match.index}`);
}
// Found "Hello" at index 0
// Found "hello" at index 13

// Search position
console.log(text.search(/world/i)); // 6 (index of first match)
console.log(text.search(/xyz/));    // -1 (not found)

// Replace
console.log(text.replace(/hello/gi, "Hi"));
// "Hi World, Hi universe"

// ===== Character Classes =====
/\d/    // Any digit (0-9)
/\D/    // Any non-digit
/\w/    // Word character (a-z, A-Z, 0-9, _)
/\W/    // Non-word character
/\s/    // Whitespace (space, tab, newline)
/\S/    // Non-whitespace
/./     // Any character except newline

// Examples
console.log("abc123".match(/\d+/));  // ["123"]
console.log("hello world".match(/\s/)); // [" "]

// ===== Character Sets =====
/[abc]/     // Match a, b, or c
/[a-z]/     // Match any lowercase letter
/[A-Z]/     // Match any uppercase letter
/[0-9]/     // Match any digit
/[a-zA-Z]/  // Match any letter
/[^abc]/    // Match anything EXCEPT a, b, c (negation)

console.log("apple".match(/[aeiou]/g)); // ["a", "e"]
console.log("abc123".match(/[^a-z]/g)); // ["1", "2", "3"]

// ===== Anchors =====
/^hello/    // Start of string
/world$/    // End of string
/\bhello\b/ // Word boundary

console.log(/^hello/.test("hello world")); // true
console.log(/^hello/.test("say hello"));   // false
console.log(/\bcat\b/.test("category"));   // false
console.log(/\bcat\b/.test("my cat"));     // true

// ===== Quantifiers =====
/a*/      // 0 or more a's
/a+/      // 1 or more a's
/a?/      // 0 or 1 a
/a{3}/    // Exactly 3 a's
/a{2,4}/  // 2 to 4 a's
/a{2,}/   // 2 or more a's

console.log("aaa".match(/a{2,}/)); // ["aaa"]
console.log("ab".match(/a+b/));    // ["ab"]

// Greedy vs Lazy
const html = "
content
";
console.log(html.match(/<.+>/));   // ["
content
"] (greedy)
console.log(html.match(/<.+?>/));  // ["
"] (lazy, add ?)

// ===== Groups =====
/(abc)/       // Capturing group
/(?:abc)/     // Non-capturing group
/(a|b)/       // Alternation (a or b)

const datePattern = /(\d{4})-(\d{2})-(\d{2})/;
const dateMatch = "2024-01-15".match(datePattern);
console.log(dateMatch[0]); // "2024-01-15" (full match)
console.log(dateMatch[1]); // "2024" (group 1)
console.log(dateMatch[2]); // "01" (group 2)
console.log(dateMatch[3]); // "15" (group 3)

// Named groups
const namedPattern = /(?\d{4})-(?\d{2})-(?\d{2})/;
const namedMatch = "2024-01-15".match(namedPattern);
console.log(namedMatch.groups.year);  // "2024"
console.log(namedMatch.groups.month); // "01"

// ===== String Methods =====

// match() - find matches
"hello world".match(/o/g); // ["o", "o"]

// matchAll() - iterator of all matches with details
const text = "test1 test2 test3";
const matches = text.matchAll(/test(\d)/g);
for (const match of matches) {
    console.log(match[0], match[1], match.index);
}
// "test1", "1", 0
// "test2", "2", 6
// "test3", "3", 12

// replace() - replace matches
"hello world".replace(/o/g, "0"); // "hell0 w0rld"

// Replace with callback
"hello world".replace(/\w+/g, (word) => {
    return word.toUpperCase();
}); // "HELLO WORLD"

// Replace with capture groups
"John Smith".replace(/(\w+) (\w+)/, "$2, $1"); // "Smith, John"

// Named group replacement
"2024-01-15".replace(
    /(?\d{4})-(?\d{2})-(?\d{2})/,
    "$/$/$"
); // "15/01/2024"

// replaceAll() - replace all (ES2021)
"hello hello".replaceAll(/hello/g, "hi"); // "hi hi"

// split() - split by pattern
"apple, banana,  orange".split(/,\s*/); 
// ["apple", "banana", "orange"]

"one1two2three3".split(/\d/);
// ["one", "two", "three", ""]

// search() - find index
"hello world".search(/world/); // 6


// ===== RegExp Methods =====

// test() - check if matches
/\d+/.test("abc123"); // true
/\d+/.test("abcdef"); // false

// exec() - detailed match info
const regex = /(\w+)@(\w+)/g;
const str = "user@domain other@site";
let result;

while ((result = regex.exec(str)) !== null) {
    console.log({
        full: result[0],
        user: result[1],
        domain: result[2],
        index: result.index
    });
}

// Email (basic)
const emailPattern = /^[\w.-]+@[\w.-]+\.\w{2,}$/;
emailPattern.test("user@example.com"); // true

// URL
const urlPattern = /^https?:\/\/[\w.-]+(?:\/[\w.-]*)*\/?$/;
urlPattern.test("https://example.com/path"); // true

// Phone number (various formats)
const phonePattern = /^[\d\s\-+()]{10,}$/;
phonePattern.test("+1 (555) 123-4567"); // true

// Password (8+ chars, letter, number, special)
const passwordPattern = /^(?=.*[A-Za-z])(?=.*\d)(?=.*[@$!%*#?&])[A-Za-z\d@$!%*#?&]{8,}$/;
passwordPattern.test("Pass@123"); // true

// Date (YYYY-MM-DD)
const datePattern = /^\d{4}-\d{2}-\d{2}$/;

// Time (HH:MM or HH:MM:SS)
const timePattern = /^\d{2}:\d{2}(:\d{2})?$/;

// Hex color
const hexColorPattern = /^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/;
hexColorPattern.test("#FFF"); // true
hexColorPattern.test("#1a2b3c"); // true

// IP address (IPv4)
const ipPattern = /^(\d{1,3}\.){3}\d{1,3}$/;

// Credit card (basic, just 16 digits)
const cardPattern = /^\d{4}[- ]?\d{4}[- ]?\d{4}[- ]?\d{4}$/;

// Slug (URL-friendly string)
const slugPattern = /^[a-z0-9]+(?:-[a-z0-9]+)*$/;

// Username (alphanumeric, underscore, 3-16 chars)
const usernamePattern = /^[a-zA-Z0-9_]{3,16}$/;

// Extract hashtags
const text = "Hello #world and #javascript";
const hashtags = text.match(/#\w+/g); // ["#world", "#javascript"]

// Extract mentions
const mentions = "@user1 and @user2".match(/@\w+/g);
// ["@user1", "@user2"]

// Extract URLs from text
const urlExtractor = /https?:\/\/[^\s]+/g;
"Visit https://example.com for more".match(urlExtractor);
// ["https://example.com"]

// Remove HTML tags
const html = "
Hello World
";
html.replace(/<[^>]+>/g, ""); // "Hello World"

// Trim whitespace
"  hello  ".replace(/^\s+|\s+$/g, ""); // "hello"

// Collapse multiple spaces
"hello   world".replace(/\s+/g, " "); // "hello world"

// ===== Lookahead =====
// Positive lookahead (?=...) - matches if followed by
/\d+(?=px)/.test("100px"); // true - digit followed by "px"
"100px 50em".match(/\d+(?=px)/g); // ["100"]

// Negative lookahead (?!...) - matches if NOT followed by
/\d+(?!px)/.test("100em"); // true
"100px 50em".match(/\d+(?!px)/g); // ["10", "50"]

// ===== Lookbehind =====
// Positive lookbehind (?<=...) - matches if preceded by
/(?<=\$)\d+/.test("$100"); // true
"$100 €200".match(/(?<=\$)\d+/g); // ["100"]

// Negative lookbehind (?.*?<\/\1>/;
htmlTag.test("
content
"); // true
htmlTag.test("
content
");   // false

// Named capture groups with (?<name>pattern)
const dateRegex = /(?\d{4})-(?\d{2})-(?\d{2})/;
const match = dateRegex.exec('2024-03-15');

// Access by name
console.log(match.groups.year);  // '2024'
console.log(match.groups.month); // '03'
console.log(match.groups.day);   // '15'

// Destructure groups
const { year, month, day } = match.groups;
console.log(`${month}/${day}/${year}`); // '03/15/2024'

// Named groups in replace
const reformatted = '2024-03-15'.replace(
    /(?\d{4})-(?\d{2})-(?\d{2})/,
    '$/$/$'
);
console.log(reformatted); // '03/15/2024'

// Named backreferences
const duplicateWords = /\b(?\w+)\s+\k\b/gi;
'the the quick brown fox fox'.replace(duplicateWords, '$');
// 'the quick brown fox'

// Complex parsing with named groups
const urlRegex = /^(?https?):\/\/(?[^\/]+)(?\/[^?]*)?(?\?.*)?$/;
const urlMatch = urlRegex.exec('https://example.com/api/users?page=1');

console.log(urlMatch.groups);
// { protocol: 'https', host: 'example.com', path: '/api/users', query: '?page=1' }

// Using with matchAll
const logRegex = /\[(?\w+)\] (?.+)/g;
const logData = '[INFO] Server started\n[ERROR] Connection failed';

for (const match of logData.matchAll(logRegex)) {
    console.log(`${match.groups.level}: ${match.groups.message}`);
}
// INFO: Server started
// ERROR: Connection failed

// Positive Lookahead (?=pattern) - must be followed by
const beforeDollar = /\d+(?=\$)/g;
'100$ 200€ 300$'.match(beforeDollar); // ['100', '300']

// Negative Lookahead (?!pattern) - must NOT be followed by
const notFollowedByDollar = /\d+(?!\$)/g;
'100$ 200€ 300$'.match(notFollowedByDollar); // ['200']

// Positive Lookbehind (?<=pattern) - must be preceded by
const afterDollar = /(?<=\$)\d+/g;
'$100 €200 $300'.match(afterDollar); // ['100', '300']

// Negative Lookbehind (?