Question

How do I search through a string for a particular substring and find the positions of each matching item.

For example,

NSString *theText = @"name - dave, age - 30, favNum - 7, name - peter, age - 8, favNum - 10";

In this example, I am trying to find all the ages and replace them with 50. What is the best way of doing so?

Answer 1

NSRegularExpression is available only since iOS 4.0 and the string should have predictable regex for this to work as one would guess.

NSString *theText = @"name - dave, age - 30, favNum - 7, name - peter, age - 8, favNum - 10";
NSRegularExpression * regularExpression = [NSRegularExpression regularExpressionWithPattern:@"(?<= age - )\\d+"
                                                                                    options:0
                                                                                      error:nil];
    
NSString * alteredText = [regularExpression stringByReplacingMatchesInString:theText
                                                                     options:0
                                                                       range:NSMakeRange(0, [theText length])
                                                                withTemplate:@"50"];

NSLog(@"%@", alteredText);

The above code works. You will have to see if it fits your HTML use case.

Getting the ages

You will use the enumerateMatchesInString:options:range:usingBlock: method to enumerate through all the matches. You will get a NSTextCheckingResult object for each iteration which will hold the range information about the match. So you will need to do this before you replace the string. You can get the ages in an array doing,

NSMutableArray * ages = [NSMutableArray array];
[regularExpression enumerateMatchesInString:theText
                                    options:0
                                      range:NSMakeRange(0, [theText length])
                                 usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop){
                                     [ages addObject:[NSNumber numberWithInt:[[theText substringWithRange:result.range] intValue]]];
                                 }];

NSLog(@"%@", ages);