A simple NSPredicate such as "username == wjl" can be parsed as a key-value pair.

NSPredicate+KeyValue.h

//
//  NSPredicate+KeyValue.h
//  IncrementalAppnet
//
//  Created by William LaFrance on 2/16/13.
//

#import <Foundation/Foundation.h>

@interface NSPredicate (KeyValue)

- (BOOL)isSimpleKeyValue;
- (NSString *)key;
- (NSString *)value;

@end

NSPredicate+KeyValue.m

//
//  NSPredicate+KeyValue.m
//  IncrementalAppnet
//
//  Created by William LaFrance on 2/16/13.
//

#import "NSPredicate+KeyValue.h"

@implementation NSPredicate (KeyValue)

- (BOOL)isSimpleKeyValue
{
    NSRegularExpression *simpleKeyValueExpression =
        [NSRegularExpression regularExpressionWithPattern:@"^[A-Za-z0-9]+ == [A-Za-z0-9]+$"
                                                  options:0
                                                    error:nil];
    assert(simpleKeyValueExpression != nil);
    
    NSTextCheckingResult *match = [simpleKeyValueExpression firstMatchInString:self.description
                                                                       options:0
                                                                         range:NSMakeRange(0, self.description.length)];
    
    if (match.range.location == 0 && match.range.length == 0) {
        return NO;
    } else {
        return YES;
    }
}

- (NSString *)key
{
    NSArray *components = [self.description componentsSeparatedByString:@" == "];
    return components[0];
}

- (NSString *)value
{
    NSArray *components = [self.description componentsSeparatedByString:@" == "];
    return components[1];
}

@end

NSPredicateKeyValueTests.h

//
//  NSPredicateKeyValueTests.h
//  IncrementalAppnet
//
//  Created by William LaFrance on 2/16/13.
//

#import <SenTestingKit/SenTestingKit.h>

@interface NSPredicateKeyValueTests : SenTestCase

@end

NSPredicateKeyValueTests.m

//
//  NSPredicateKeyValueTests.m
//  IncrementalAppnet
//
//  Created by William LaFrance on 2/16/13.
//

#import "NSPredicateKeyValueTests.h"
#import "NSPredicate+KeyValue.h"

@implementation NSPredicateKeyValueTests

- (void)testIsSimpleKeyValue
{
    NSPredicate *p1 = [NSPredicate predicateWithFormat:@"fizz == buzz"];
    NSPredicate *p2 = [NSPredicate predicateWithFormat:@"fizz == buzz && foobar == bazquux"];
    
    STAssertTrue(p1.isSimpleKeyValue, nil);
    STAssertFalse(p2.isSimpleKeyValue, nil);
}

- (void)testKey
{
    NSPredicate *p1 = [NSPredicate predicateWithFormat:@"fizz == buzz"];
    NSPredicate *p2 = [NSPredicate predicateWithFormat:@"foo == bar"];
    
    STAssertEqualObjects(p1.key, @"fizz", nil);
    STAssertEqualObjects(p2.key, @"foo", nil);
}

- (void)testValue
{
    NSPredicate *p1 = [NSPredicate predicateWithFormat:@"fizz == buzz"];
    NSPredicate *p2 = [NSPredicate predicateWithFormat:@"foo == bar"];
    
    STAssertEqualObjects(p1.value, @"buzz", nil);
    STAssertEqualObjects(p2.value, @"bar", nil);
}

@end