Stringconversion into allowed characters

I need to check, if a string contains only allowed characters.

Allowed characters are A-Z, a-z, 0-9

In case a non allowed character (e.g. ü, é etc) is contained, I need to replace it by an allowed character
Ü becomes ue, é becomes e and so on.

Is there a generic way to achieve this without checking each character against the allowed characters and define the mapping for each non allowed character?