parse-time-string.js

function parse(str) {

	let tmp = str.trim().toUpperCase()
  tmp = tmp.replaceAll(" ", "")
  tmp = tmp.replaceAll(":", "")
    
  let matches = tmp.match(/([0-9]{1,4})(AM|PM)/g)
  
  if (matches.length === 0) {
  	return
  }
  
  let match = matches[0].split('')
  // match= 3PM, 30PM, 300PM, 3000PM

  let timeOfDay = match.splice(-2).join('') // AM or PM
  
  // match= 3, 30, 300, 3000

  let hr
  let min

  switch(match.length) {
    case 1:
    case 2:
      hr = match.join('')
      min = "00"
      break;
    case 3:
      hr = match.slice(0, 1).join('')
      min = match.slice(1, 3).join('')
      break
    case 4:
      hr = match.slice(0, 2).join('')
      min = match.slice(2, 4).join('')
      break
  } 

  if (hr >= 1 && hr <= 12 && min >= 0 && min <= 59) {
    return `${hr}:${min} ${timeOfDay}`
  }
             
}

console.log('3pm', parse('3pm'))
console.log('12pm', parse('12pm'))
console.log('13pm', parse('13pm'))
console.log('315am', parse('315am'))
console.log('1215am', parse('1215am'))
console.log('11:45am', parse('11:45am'))
console.log('3015am', parse('3015am'))
console.log('jk2j4pm', parse('jk2j4pm'))