regexp.js

const isNotValidRE = /[^A-Za-z]/;
const matchRLE = /([A-Za-z])\1*/g;

function RLE(str) {
  if (str && isNotValidRE.test(str)){
    throw new Error('Not valid string: ' + str);
  }
  return str.replace(
    matchRLE,
    match => match.length === 1 ? match : `${match[0]}${match.length}`
  );
}

try {
  RLE('AS-');
} catch (e) {
   console.log('should throw %s', e.message);
}

console.log('empty str %s', RLE('') === '');
console.log('single char %s', RLE('A') === 'A');
console.log('couple char %s', RLE('Aa') === 'Aa');
console.log('couple of diffchar %s', RLE('AB') === 'AB');
console.log('real test %s', RLE('AAAABBQWEEEE') === 'A4B2QWE4');