在 iphone 上使用正则表达式捕获括号

Question

Trying to get the URL out of some HTML I'm parsing (on iPhone) using 'capturing parentheses' to just group the bit I'm interested in.

I now have this:

NSString *imageHtml;  //a string with some HTML in it

NSRegularExpression* innerRegex = [[NSRegularExpression alloc] initWithPattern:@"href=\"(.*?)\"" options:NSRegularExpressionCaseInsensitive|NSRegularExpressionDotMatchesLineSeparators error:nil];
NSTextCheckingResult* firstMatch = [innerRegex firstMatchInString:imageHtml options:0 range:NSMakeRange(0, [imageHtml length])];
[innerRegex release];

if(firstMatch != nil)
{
    newImage.detailsURL = 
    NSLog(@"found url: %@", [imageHtml substringWithRange:firstMatch.range]);
}

The only thing it lists is the full match (so: href="http://tralalala.com" instead of http://tralalala.com

How can I force it to only return my first capturing parentheses match?

Answer 1

Regex groups work by capturing the whole match in group 0, then all groups in the regex will start at index 1. NSTextCheckingResult stores these groups as ranges. Since your regex requires at least one group the following will work.

NSString *imageHtml = @"href=\"http://tralalala.com\"";  //a string with some HTML in it

NSRegularExpression* innerRegex = [[NSRegularExpression alloc] initWithPattern:@"href=\"(.*?)\"" options:NSRegularExpressionCaseInsensitive|NSRegularExpressionDotMatchesLineSeparators error:nil];
NSTextCheckingResult* firstMatch = [innerRegex firstMatchInString:imageHtml options:0 range:NSMakeRange(0, [imageHtml length])];
[innerRegex release];

if(firstMatch != nil)
{
    //The ranges of firstMatch will provide groups, 
    //rangeAtIndex 1 = first grouping
    NSLog(@"found url: %@", [imageHtml substringWithRange:[firstMatch rangeAtIndex:1]]);
}

Answer 2

You need pattern something like this:

(?<=href=\")(.*?)(?=\")