CHARACTER :: string = 'ABCDEFGHIJK', known = 'B',  substring = 'CDE'
REAL, PARAMETER :: n = 5,  m = 8

WRITE(Messagebox) string(n : n + m - 1), "| substring starting from n, length m"
WRITE(Messagebox) string(n :), "| substring starting from n, to  end of string"
WRITE(Messagebox) string(1: LEN(string)-1), "| whole string minus last character"

pos_known = INDEX(string, known)
WRITE(Messagebox) string(pos_known : pos_known+m-1), "| substring starting from pos_known, length m"

pos_substring = INDEX(string, substring)
WRITE(Messagebox) string(pos_substring : pos_substring+m-1), "| substring starting from pos_substring, length m"