Я понял!
Это код (написано не очень красиво):
NSRange rr2 = [content rangeOfString:@"href=\""];
NSRange rr3 = [content rangeOfString:@"\" />"];
int lengt = rr3.location - rr2.location - rr2.length;
int location = rr2.location + rr2.length;
NSRange aa;
aa.location = location;
aa.length = lengt;
theString = [theString substringWithRange:aa];