전화번호 포맷팅 — replace 는 매치된 부분만 바꾸고 나머지는 뒤에 그대로 남긴다. 그래서 그룹 자릿수 합이 길이와 같아야 한다¹

function formatPhoneNumber(phoneNumber) {
  const cleaned = phoneNumber.replace(/\D/g, '')

  if (cleaned.length === 11) {
    return cleaned.replace(/(\d{3})(\d{4})(\d{4})/, '$1-$2-$3')
  } else if (cleaned.length === 10 && cleaned.startsWith('02')) {
    return cleaned.replace(/(\d{2})(\d{4})(\d{4})/, '$1-$2-$3')
  } else if (cleaned.length === 10) {
    return cleaned.replace(/(\d{3})(\d{3})(\d{4})/, '$1-$2-$3')
  } else if (cleaned.length === 9 && cleaned.startsWith('02')) {
    return cleaned.replace(/(\d{2})(\d{3})(\d{4})/, '$1-$2-$3')
  }
  return 'Invalid phone number'
}

formatPhoneNumber('01012341234') // 010-1234-1234
formatPhoneNumber('021234567') // 02-123-4567
formatPhoneNumber('0212345678') // 02-1234-5678

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Footnotes

1. String.prototype.replace() - MDN: “returns a new string with one, some, or all matches of a pattern replaced by a replacement.” ↩