title: How to search a subString from a NSString with case-insensitive
body: <p>I knew a instance method of NSString.</p>
  
  <pre><code>- (NSRange)rangeOfString:(NSString *)aString
  </code></pre>
  
  <p>My source string looks like this: "abcdEFGhi"</p>
  
  <p>When I use <code>[srcStr.rangeOfString:@"efg"];</code>, there is no substring was found.</p>
  
  <p>How can I solve this problem?</p>