書き直しです。
buff = "0123456おにたま\nabcde"
A = strmid(buff, 0, 7)
i1 = instr(buff, strlen(A), "\n")
B = strmid(buff, strlen(A), i1)
i2 = strlen(buff) - strlen(A) + strlen(B) + strlen("\n")
C = strmid(buff, strlen(A) + strlen("\n") + i1, i2)
mes A
mes B
mes C