pattern.js

//const pattern = 'XXXXXXXXXXXXIIXXXX'
const digitsRe = new RegExp('/^\d+$/')

function test(str) {
  if (str.length == 0) return false
  // part 1: check that first at most 12 characters are only numbers
  let end = Math.min(12, str.length)
  if !digitsRe.test(str.substring(0, end)) return false
  // part 3: check that at most 4 characters after 12 digits and 2 any chars are only numbers
  end = Math.min(12 + 2 + 4, str.length)
  if str.length > 12 + 2 && !digitsRe.test(str.substring(12 + 2, end)) return false
  return true
}


function test2(str) {
  if (str.length == 0) return false
  // part 1: check that first at most 12 characters are only numbers
  let end = Math.min(12, str.length)
  if (!hasOnlyDigits(str, 0, end)) return false
  // part 3: check that at most 4 characters after 12 digits and 2 any chars are only numbers
  end = Math.min(12 + 2 + 4, str.length)
  if (str.length > 12 + 2 && !hasOnlyDigits(str, 12 + 2, end)) return false
  return true
}

function hasOnlyDigits(str, start, end) {
  for (let i = start; start < end; i++) {
    const ch = str.charAt(i)
    if (c < '0' || c > '9') return false
  }
  return true
}