iPhone - Replacing line breaks in my TextView with <br />?

I am trying to convert lineBreaks in my UITextView to <br /> tags, why does the following line crash? It complains about [NSCharacterSet newlineCharacterSet] because when I pass it a normal string it works fine

[myMutableString stringByReplacingOccurrencesOfString:
  [NSCharacterSet newlineCharacterSet] withString:@"<br />"];

Terminating app due to uncaught exception 'NSInvalidArgumentException', 开发者_开发知识库
reason: '-[__NSCFCharacterSet length]: unrecognized selector sent to 
instance 0x7b1cc80'

A NSCharacterSet is not a NSString.So you're passing a wrong argument. If you have only newline "\n" characters you can use:

[myMutableString stringByReplacingOccurrencesOfString:@"\n" 
                                           withString:@"<br />"];

If you have also "\r" and "\lf" you can try something like this (untested)

NSRange range;
while((range = [myMutableString rangeOfCharacterFromSet:
                [NSCharacterSet newlineCharcaterSet]]).location != NSNotFound) {
    [myMutableString replaceCharactersInRange:range withString:@"<br />"];
}