<iframe width="1280" height="720" src="http://www.youtube.com/embed/Pb55ep-DrSo?wmode=opaque&autoplay=1&fs=1&feature=oembed&showinfo=0&autohide=1&controls=0" frameborder="0" allowfullscreen></iframe>

NSString* str = @"<iframe width="1280" height="720" src="http://www.youtube.com/embed/Pb55ep-DrSo?wmode=opaque&autoplay=1&fs=1&feature=oembed&showinfo=0&autohide=1&controls=0" frameborder="0" allowfullscreen></iframe>";
str = [str substringFromIndex:[str rangeOfString:@"src=""].location+[str rangeOfString:@"src=""].length];
str = [str substringToIndex:[str rangeOfString:@"""].location ];
NSLog(@"Str %@",str);

2012-08-17 09:16:55.285 TEST[24413:c07] Str http://www.youtube.com/embed/Pb55ep-DrSo?wmode=opaque&autoplay=1&fs=1&feature=oembed&showinfo=0&autohide=1&controls=0

src[s]*=[s]*"([^"]*)"

NSString *searchedString = @"<iframe width="1280" height="720" src="http://www.youtube.com/embed/Pb55ep-DrSo?wmode=opaque&autoplay=1&fs=1&feature=oembed&showinfo=0&autohide=1&controls=0" frameborder="0" allowfullscreen></iframe>";
NSError* error = nil;

NSRegularExpression* regex = [NSRegularExpression regularExpressionWithPattern:@"src[s]*=[s]*"([^"]*)"" options:0 error:&error];
NSArray* matches = [regex matchesInString:searchedString options:0 range:NSMakeRange(0, [searchedString length])];
for ( NSTextCheckingResult* match in matches )
{
    NSString* matchText = [searchedString substringWithRange:[match range]];
    NSLog(@"match: %@", matchText);
    NSRange group1 = [match rangeAtIndex:1];
    NSLog(@"group1: %@", [searchedString substringWithRange:group1]);
}