al_ustr_prev - Allegro 5 API

       Find  the byte offset of the previous code point in string, before *pos.  *pos does not have to be at the
       beginning of a code point.  Returns true on success, and the value pointed to by pos will be  updated  to
       the  found  offset.   Otherwise  returns  false if *pos was already at the end of the string, and *pos is
       unmodified.

       This function just looks for an appropriate byte; it doesn’t check if found offset is the beginning of  a
       valid  code  point.   If you are working with possibly invalid UTF-8 strings then it could skip over some
       invalid bytes.

       al_ustr_prev - Allegro 5 API

al_ustr_next(3alleg5)

Allegro reference manual                                                                   al_ustr_prev(3alleg5)

              #include <allegro5/allegro.h>

              bool al_ustr_prev(const ALLEGRO_USTR *us, int *pos)