I want to write a lisp that read a text from model space and replace some characters in it. for this work i iterate each character and take Ascii code of each one, then in if-else do replacement. in some chars it get wrong answers. for example in character "Þ" (latin capital letter thorn)(Ascii code:222 or Hex=\U+00DE). 
both (vl-string-elt "Þ" 0) and (Ascii "Þ") commands give me 92 as a resault of it ascii code  
i use doslib, but same problem exist. 
does any one know what's the problem?