Unescape a JSON string.

KDJ_stringWthJSONString_Tests.m

#import <SenTestingKit/SenTestingKit.h>
#import "NSString+KDJ_stringWthJSONString.h"

@interface KDJ_stringWthJSONString_Tests : SenTestCase
@end

@implementation KDJ_stringWthJSONString_Tests

- (void)test_stringFromJSONString {
    STAssertEqualObjects(@"", [NSString stringWithJSONString:@"\"\""], @"empty string");
    STAssertEqualObjects(@"foo", [NSString stringWithJSONString:@"\"foo\""], @"simple string with no escaped characters");
    STAssertEqualObjects(@"\"", [NSString stringWithJSONString:@"\"\\\"\""], @"escaped double-quote character");
    STAssertEqualObjects(@"\\", [NSString stringWithJSONString:@"\"\\\\\""], @"escaped backslash");
    STAssertEqualObjects(@"/", [NSString stringWithJSONString:@"\"\\/\""], @"escaped forward slash");
    STAssertEqualObjects(@"\b", [NSString stringWithJSONString:@"\"\\b\""], @"escaped backspace");
    STAssertEqualObjects(@"\f", [NSString stringWithJSONString:@"\"\\f\""], @"escaped form feed");
    STAssertEqualObjects(@"\n", [NSString stringWithJSONString:@"\"\\n\""], @"escaped line feed");
    STAssertEqualObjects(@"\r", [NSString stringWithJSONString:@"\"\\r\""], @"escaped carriage return");
    STAssertEqualObjects(@"\t", [NSString stringWithJSONString:@"\"\\t\""], @"escaped tab");

    STAssertEqualObjects(@" ", [NSString stringWithJSONString:@"\"\\u0020\""], @"escaped Unicode");
    STAssertEqualObjects(@"zFoo", [NSString stringWithJSONString:@"\"\\u007aFoo\""], @"string with escaped Unicode");

    STAssertEqualObjects(@"She said, \"Hello!\"\n\tThen she said \"Goodbye.\"",
                         [NSString stringWithJSONString:@"\"She said, \\\"Hello!\\\"\\n\\tThen she said \\\"Goodbye.\\\"\""],
                         @"Complicated string");

    STAssertNil([NSString stringWithJSONString:@""], @"must start and end with double-quote characters");
    STAssertNil([NSString stringWithJSONString:@"foo"], @"must start and end with double-quote characters");
    STAssertNil([NSString stringWithJSONString:@"\"foo"], @"must start and end with double-quote characters");
    STAssertNil([NSString stringWithJSONString:@"foo\""], @"must start and end with double-quote characters");
    STAssertNil([NSString stringWithJSONString:@"\"foo\\\""], @"cannot end with backslash");
    STAssertNil([NSString stringWithJSONString:@"\"\\u123"], @"four characters must follow \\u");
}

@end

NSString+KDJ_stringWthJSONString.h

#import <Foundation/Foundation.h>

@interface NSString (KDJ_stringWithJSONString)

// Return string from a JSON-encoded string, or return nil if argument is not a valid JSON string value.
//
// See RFC 4627 section 2.5 for details of JSON string representation.
// The string must begin and end with double-quote (") characters.

+ (NSString *)stringWithJSONString:(NSString *)jsonString;

@end

NSString+KDJ_stringWthJSONString.m

#import "NSString+KDJ_stringWithJSONString.h"

@implementation NSString (KDJ_stringWithJSONString)

+ (NSString *)stringWithJSONString:(NSString *)jsonString {
    NSUInteger len = [jsonString length];
    if (len < 2 || [jsonString characterAtIndex:0] != '"' || [jsonString characterAtIndex:(len - 1)] != '"') {
        NSLog(@"%s: string does not begin and end with \" characters", __PRETTY_FUNCTION__);
        return nil;
    }

    NSMutableString *result = [[NSMutableString alloc] initWithCapacity:(len-2)];
    NSUInteger limit = len - 1;
    for (NSUInteger i = 1; i < limit; ++i) {
        unichar character = [jsonString characterAtIndex:i];

        if (character == '\\') {

            // Escape sequence

            if (i == limit - 1) {
                NSLog(@"%s: JSON string cannot end with single backslash", __PRETTY_FUNCTION__);
                return nil;
            }

            ++i;
            unichar nextCharacter = [jsonString characterAtIndex:i];
            switch (nextCharacter) {
                case 'b':
                    [result appendString:@"\b"];
                    break;

                case 'f':
                    [result appendString:@"\f"];
                    break;

                case 'n':
                    [result appendString:@"\n"];
                    break;

                case 'r':
                    [result appendString:@"\r"];
                    break;

                case 't':
                    [result appendString:@"\t"];
                    break;

                case 'u':
                    if ((i + 4) >= limit) {
                        NSLog(@"%s: insufficient characters remaining after \\u in JSON string", __PRETTY_FUNCTION__);
                        return nil;
                    }
                    {
                        NSString *hexdigits = [jsonString substringWithRange:NSMakeRange(i + 1, 4)];
                        i += 4;
                        NSScanner *scanner = [NSScanner scannerWithString:hexdigits];
                        unsigned int hexValue = 0;
                        if (![scanner scanHexInt:&hexValue]) {
                            NSLog(@"%s: invalid hex digits following \\u", __PRETTY_FUNCTION__);
                        }
                        [result appendFormat:@"%C", (unichar)hexValue];
                    }
                    break;

                default:
                    [result appendFormat:@"%C", nextCharacter];
                    break;
            }
        }
        else {
            // No escape
            [result appendFormat:@"%C", character];
        }
    }

    // Return an immutable copy
    return [NSString stringWithString:result];
}

@end

It turns out that this code is unnecessary. See http://stackoverflow.com/questions/16948427/how-can-you-deserialize-an-escaped-json-string-with-nsjsonserialization