If the first character in from_string occurs in the source_string, all occurrences of it are replaced by the first character in to_string. This repeats for all characters in from_string and for all characters in to_string. The replacement is performed character-by-character, that is, the replacement of the second character is done on the string resulting from the replacement of the first character.
If from_string contains more characters than to_string, the extra characters are removed from the source_string.
If from_string contains fewer characters than to_string, the extra characters in to_string have no effect.
If the same character occurs more than once in from_string, only the replacement character from the to_string corresponding to the first occurrence is used.