Regex

// literal — use when pattern is fixed
const re = /hello/i
 
// constructor — use when pattern is dynamic
const re2 = new RegExp(`^${userInput}$`, 'i')

/pattern/g   // global — all matches, not just first
/pattern/i   // case insensitive
/pattern/m   // multiline — ^ $ match line starts/ends
/pattern/s   // dotAll — . matches newlines too
/pattern/gi  // combine flags

/\d/     // digit [0-9]
/\D/     // not a digit
/\w/     // word char [a-zA-Z0-9_]
/\W/     // not a word char
/\s/     // whitespace (space, tab, newline)
/\S/     // not whitespace
/./      // any char except newline

/^start/    // start of string
/end$/      // end of string
/\bword\b/  // word boundary
/\Bword\B/  // not a word boundary
 
'hello world'.match(/\bworld\b/) // 'world'
'helloworld'.match(/\bworld\b/)  // null

/a*/     // 0 or more
/a+/     // 1 or more
/a?/     // 0 or 1
/a{3}/   // exactly 3
/a{2,5}/ // 2 to 5
/a{2,}/  // 2 or more
 
/a+?/    // lazy — match as few as possible

/(abc)/      // capturing group
/(?:abc)/    // non-capturing group
/(?<name>x)/ // named group
 
/cat|dog/    // either cat or dog
/(red|blue) car/ // red car or blue car

/\d+(?= dollars)/   // lookahead: digits before " dollars"
/\d+(?! dollars)/   // negative: NOT before " dollars"
/(?<=\$)\d+/        // lookbehind: digits after "$"
/(?<!\$)\d+/        // negative: NOT after "$"

// returns true/false — fastest for checking
/\d+/.test('abc123')  // true
/^[a-z]+$/.test('Hi') // false (has uppercase)
 
if (/^\d{5}$/.test(zip)) {
  // valid 5-digit zip
}

// returns first match (or null)
'hello 42 world 99'.match(/\d+/)
// ['42', index: 6]
 
// with /g — returns all matches
'hello 42 world 99'.match(/\d+/g)
// ['42', '99']

// replace first
'hello world'.replace(/world/, 'JS')
// 'hello JS'
 
// replace all
'aabbcc'.replace(/b/g, 'X')
// 'aaXXcc'
 
// with capture group
'John Smith'.replace(/(\w+) (\w+)/, '$2, $1')
// 'Smith, John'

// split on pattern
'one, two,  three'.split(/,\s*/)
// ['one', 'two', 'three']
 
// split on multiple delimiters
'a.b-c_d'.split(/[.\-_]/)
// ['a', 'b', 'c', 'd']

const email = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
 
email.test('a@b.com')  // true
email.test('not@real')  // false

const hex = /^#([0-9a-f]{3}|[0-9a-f]{6})$/i
 
hex.test('#fff')    // true
hex.test('#1a2b3c') // true
hex.test('#gggggg') // false

const str = 'age: 25, score: 100'
const matches = [...str.matchAll(/(\w+): (\d+)/g)]
 
matches[0][0] // 'age: 25'   (full match)
matches[0][1] // 'age'        (group 1)
matches[0][2] // '25'         (group 2)

const html = '<img src="cat.png" alt="a cat" width="100">'
 
const values = [...html.matchAll(/\w+="([^"]*)"/g)]
  .map(m => m[1])
// ['cat.png', 'a cat', '100']

const html = '<li>Apple</li><li>Banana</li><li>Cherry</li>'
 
const items = [...html.matchAll(/<li>(.*?)<\/li>/g)]
  .map(m => m[1])
// ['Apple', 'Banana', 'Cherry']

const text = 'Visit https://a.com and http://b.io/path today'
 
const urls = text.match(/https?:\/\/[^\s]+/g)
// ['https://a.com', 'http://b.io/path']

const config = 'host=localhost;port=5432;db=myapp'
 
const pairs = [...config.matchAll(/(\w+)=([^;]+)/g)]
  .map(m => [m[1], m[2]])
 
Object.fromEntries(pairs)
// { host: 'localhost', port: '5432', db: 'myapp' }

// strip HTML tags
'<p>Hello <b>world</b></p>'.replace(/<[^>]*>/g, '')
// 'Hello world'
 
// collapse whitespace
'hello    world'.replace(/\s+/g, ' ')
// 'hello world'

const str = 'name: Alice, age: 30, role: admin'
 
const entries = [...str.matchAll(/(?<key>\w+): (?<val>\w+)/g)]
  .map(m => [m.groups.key, m.groups.val])
 
Object.fromEntries(entries)
// { name: 'Alice', age: '30', role: 'admin' }

const md = 'See [Google](https://google.com) and [MDN](https://mdn.io)'
 
const links = [...md.matchAll(/\[([^\]]+)\]\(([^)]+)\)/g)]
  .map(m => ({ text: m[1], url: m[2] }))
// [{ text: 'Google', url: 'https://google.com' },
//  { text: 'MDN', url: 'https://mdn.io' }]

function slugify(str) {
  return str
    .toLowerCase()
    .replace(/[^a-z0-9]+/g, '-')
    .replace(/^-|-$/g, '')
}
 
slugify('Hello World!') // 'hello-world'

function toKebab(str) {
  return str.replace(/([a-z])([A-Z])/g, '$1-$2')
    .toLowerCase()
}
 
toKebab('backgroundColor') // 'background-color'
toKebab('fontSize')        // 'font-size'

const strong = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%]).{8,}$/
 
// requires: lowercase, uppercase, digit,
// special char, min 8 chars
strong.test('Passw0rd!')  // true
strong.test('password')   // false

const line = 'Alice,"New York, NY",30'
 
const fields = [...line.matchAll(/("(?:[^"\\]|\\.)*"|[^,]*)/g)]
  .map(m => m[1].replace(/^"|"$/g, ''))
// ['Alice', 'New York, NY', '30']