在RegEx标识的NSString中的某个点添加一个字符，是否有比所有后向引用更有效的方法？

Question

I want to take a URL like http://imgur.com/isodf99.jpg add the letter m right before the file extension. So that URL before becomes http://imgur.com/isodf99m.jpg .

I'm guessing the best way to identify that point would be via RegEx (I know I could just go back four places from the end, but that wouldn't work with .jpeg ) but I'm having trouble figuring out the best implementation of it. I'm confused how to say "find that string of random letters and numbers and add an m to the end of it".

My first reaction was to separate the portions of the URL into "before code", "code" and "after code" and save them as back references, then reconstruct them but add an m to the second back reference.

So, this screenshot for example illustrates it:

(The app is Patterns by the way, always get asked that.)

But I'm not sure that's the best way. Is there a more straightforward way?

Answer 1

If you don't insist on using regular expressions, this would work:

NSString *url = @"http://imgur.com/isodf99.jpg";
url = [[[url stringByDeletingPathExtension]
        stringByAppendingString:@"m"]
       stringByAppendingPathExtension:[url pathExtension]];
NSLog(@"%@", url);
// http://imgur.com/isodf99m.jpg

Or, if you work with NSURL instead of NSString:

NSURL *url = [NSURL URLWithString:@"http://imgur.com/isodf99.jpg"];

// Extract the path:
NSString *path = [url path];

// Insert "m" before path extension:
path = [[[path stringByDeletingPathExtension]
        stringByAppendingString:@"m"]
        stringByAppendingPathExtension:[path pathExtension]];

// Rebuild URL with new path:
url = [[NSURL alloc] initWithScheme:[url scheme] host:[url host] path:path];

But if you prefer a regular expression:

NSString *url = @"http://imgur.com/isodf99.jpeg";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"(\\.[^.]*)$"
                                                                       options:0 error:NULL];
url = [regex stringByReplacingMatchesInString:url
                                      options:0
                                        range:NSMakeRange(0, [url length])
                                 withTemplate:@"m$1"];