*   *   😀  A   B   😀  |   😀  A   B   
0   1   2   6   7   8   12  13  17  18  

find_first_char ==> index=6, char='A'
find_first_char ==> index=2, char='😀'
find_first_char failed

find_last_char ==> index=17, char='A'
find_last_char ==> index=13, char='😀'
find_last_char failed