In [1]:
import re

def string_to_number(string):
    numbers = []
    for char in string:
        if re.match(r'[A-Za-z]', char):
            numbers.append(f"{ord(char):03d}")
            continue
        if bool(re.match(r'\d', char)):
            numbers.append(f"{char:1s}")
            continue
        return "Unknown character"
    return "".join(numbers)
In [2]:
string_to_number("hallø")
Out[2]:
'Unknown character'
In [3]:
string_to_number("halla")
Out[3]:
'104097108108097'