Thank you for pointing to this useful function: unicode_idx.

Great! I'm looking forward this improvement.

As you are working on the UTF-8 implementation, I mention here another small thing I saw, concerning indexing in strings.
I'm curious to here your opinion about this.

This problem is independent of the encoding !
Let's consider and execute the following code:

str1='12345.03 cm, 12345.03 cm';
str2='12345.04 µm, 12345.03 µm';

length(str1)
length(str2)

ans = 24
ans = 26

Although the two strings have the same length in terms of characters, Octave returns the length in terms of bytes.

Now if I want to get the 2nd occurence of the units (cm or µm) contained in the strings I have to select different start position but also different size, i.e. the size is dependent on the content and it is likely I have to decode UTF-8 myself to be able to isolate correctly a particular character.

str1(23 : 23 + 1)
str2(24 : 24 + 1)
str2(24 : 24 + 2)

ans = cm
ans = µ
ans = µm

Note that the same behaviour occurs in Matlab. However, due to the UTF-16 encoding this case occurs probabely very rarely, (not a problem for the above example).
An example of a Matlab string with a character coded with 2*16bits is given heraafter.

   test_str_for_ML=['1234', char(hex2dec('D834')), char(hex2dec('DD1E')),',7890']

Editing text in the command window is unstable when special characters are used

This is independent on the encoding !

Type following things in the console and see what happens for each case

  A) disp('é')  then Enter                             -> Everything is ok

  B) disp('é')  then 4 time BS  then 'a')  then Enter  -> Observe the result

         >> disp('a')
         error: parse error:

           syntax error

         >>> disp(''a')        
                          ^
                         
         i.e. The executed code is not the code display in the prompt
        
                
  C) disp('é') then 4 time left arrow  then a  then Enter  -> Observe the result

         >> disp(aé')
         aé

         i.e.  aé is displayed although the command appearing in the console was invalid

     Then press up arrow and the previously executed command is displayed (different than what was displayed)

 etc. etc.

 - sometimes the display is also disabled as posted previously