import re


def replace_last_number(input_string, replace_string):
    """
    Replace the last number with something.
    
    Args:
        input_string (str): A string to search for the last number.
        replace_string (str): The string to replace the last number with.
        
    Returns:
        str: The new string after replacing.
    """
    
    replace_string = str(replace_string)
    
    expression = re.compile('(\d+)(?=(\D+)?$)')
    search = expression.search(input_string)

    if not search:
        return input_string + replace_string
    
    #count = len(search.group())
    
    #replace_count = len(replace_string)
    
    #if replace_count == 1:
    #    replace_string *= count
    
    #if replace_count:
    return input_string[:search.start()] + replace_string + input_string[search.end():]