Getting wrong range with regex in Objective-C

Question

I have the following string: "Title: " and I am using regex in Objective-C to extract the "/books/1/title" part from the string. (The string can contain multiple expressions) The regex is as follows <\?(.+?)\?>. My problem is that it matches the whole string(from index 0 to 24) and not the content between the tags. The code is as follows:

NSString *object = @"Title: <?/books/1/title?>";

NSMutableString *newString = [[NSMutableString alloc] initWithString:object];

NSString *pattern = @"<\?(.+?)\?>";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:pattern
                                                                           options:0 error:NULL];

NSArray *matches = [regex matchesInString:object options:0 range:NSMakeRange(0, [object length])];

for (NSInteger i = [matches count]-1; i>=0 ; i--) {
    NSTextCheckingResult * match = [matches objectAtIndex:i];

    if (match != nil && [match rangeAtIndex:0].location != NSNotFound && [match numberOfRanges] == 2) {

        NSRange part1Range = [match rangeAtIndex:1];
        NSLog(@"%lu %lu", (unsigned long)part1Range.location, (unsigned long)part1Range.length);
    }
}

Sergey Kalinichenko · Accepted Answer · 2014-06-17 11:33:35Z

3

It looks like you have incorrectly escaped the question marks: you used a single backslash, but Objective-C compiler needs two slashes in a string literal in order to represent a single backslash:

NSString *pattern = @"<\\?(.+?)\\?>";

Without the extra slash single slashes are not becoming part of the string, so regex engine sees this expression <?(.+?)?>, treats the opening angular bracket as optional, and then proceeds to matching the whole text up to the closing angular bracket.

One way to escape meta-characters, such as question marks and dots, is enclosing them into a character class instead of using a backslash. This expression is equivalent to yours, but it does not additional escaping of slashes:

NSString *pattern = @"<[?](.+?)[?]>";

answered Jun 17, 2014 at 11:33

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Alan Ihre Over a year ago

Solved my problem, thank you very much for a good solution and explanation.

zx81 Over a year ago

Hard to debug unless you know how Objective-C requires regex strings (I don't!) +1 :)

Collectives™ on Stack Overflow

Getting wrong range with regex in Objective-C

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related