al_ustr_prev - Allegro 5 API
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#include <allegro5/allegro.h>
bool al_ustr_prev(const ALLEGRO_USTR *us, int *pos)
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.