NSURL pull out a single value for a key in a parameter string

UPDATE:

Since 2010 when this was written, it seems Apple has released a set of tools for that purpose. Please see the answers below for those.

Old-School Solution:

Well I know you said “the quickest way” but after I started doing a test with NSScanner I just couldn’t stop. And while it is not the shortest way, it is sure handy if you are planning to use that feature a lot. I created a URLParser class that gets these vars using an NSScanner. The use is a simple as:

URLParser *parser = [[[URLParser alloc] initWithURLString:@"http://blahblahblah.com/serverCall?x=a&y=b&z=c&flash=yes"] autorelease];
NSString *y = [parser valueForVariable:@"y"];
NSLog(@"%@", y); //b
NSString *a = [parser valueForVariable:@"a"];
NSLog(@"%@", a); //(null)
NSString *flash = [parser valueForVariable:@"flash"];
NSLog(@"%@", flash); //yes

And the class that does this is the following (*source files at the bottom of the post):

URLParser.h

@interface URLParser : NSObject {
    NSArray *variables;
}

@property (nonatomic, retain) NSArray *variables;

- (id)initWithURLString:(NSString *)url;
- (NSString *)valueForVariable:(NSString *)varName;

@end

URLParser.m

@implementation URLParser
@synthesize variables;

- (id) initWithURLString:(NSString *)url{
    self = [super init];
    if (self != nil) {
        NSString *string = url;
        NSScanner *scanner = [NSScanner scannerWithString:string];
        [scanner setCharactersToBeSkipped:[NSCharacterSet characterSetWithCharactersInString:@"&?"]];
        NSString *tempString;
        NSMutableArray *vars = [NSMutableArray new];
        [scanner scanUpToString:@"?" intoString:nil];       //ignore the beginning of the string and skip to the vars
        while ([scanner scanUpToString:@"&" intoString:&tempString]) {
            [vars addObject:[tempString copy]];
        }
        self.variables = vars;
        [vars release];
    }
    return self;
}

- (NSString *)valueForVariable:(NSString *)varName {
    for (NSString *var in self.variables) {
        if ([var length] > [varName length]+1 && [[var substringWithRange:NSMakeRange(0, [varName length]+1)] isEqualToString:[varName stringByAppendingString:@"="]]) {
            NSString *varValue = [var substringFromIndex:[varName length]+1];
            return varValue;
        }
    }
    return nil;
}

- (void) dealloc{
    self.variables = nil;
    [super dealloc];
}

@end

*if you don’t like copying and pasting you can just download the source files – I made a quick blog post about this here.