// Str = @"PEK_LHR"
NSRange range = [trafficeStr rangeOfString:@"_" options:NSBackwardsSearch];
from = [trafficeStr substringToIndex:range.location];
to = [trafficeStr substringFromIndex:range.location + 1];
原文：http://www.cnblogs.com/lihaibo-Leao/p/4153176.html