```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') throw new Error('Input must be a string');

  let encoded = '';
  let count = 1;
  for (let i = 0; i < input.length; i++) {
    if (input[i] === input[i + 1]) {
      count++;
    } else {
      const charCode = input.charCodeAt(i);
      if (charCode >= 48 && charCode <= 57) { // Check if the next character is a digit
        encoded += `${input[i]}${count}`;
      } else {
        encoded += `${input[i]}1`;
      }
      count = 1;
    }
  }

  return encoded.replace(/1/g, ''); // Remove trailing '1's
}
```