Try this example:
[dad]$ tclsh
% set yo "Athens u0391u03B8u03AEu03BDu03B1u03B9"
Athens ??????
% set c [string length $yo]
13
% set c [string bytelength $yo]
19
If you have wish and the correct fonts installed then you can see what
the question marks really are by typing:
[dad]$ wish
% set yo "Athens u0391u03B8u03AEu03BDu03B1u03B9"
Athens ??????
% pack [text .t]
% .t insert end $yo