regular expression to match " but not \" - iphone

Question

How can I construct a regular expression which matches an literal " but only if it is not preceded by the escape slash namely \
I have a NSMutableString str which prints the following on NSLog. The String is received from a server online.
"Hi, check out \"this book \". Its cool"
I want to change it such that it prints the following on NSLog.
Hi, check out "this book ". Its cool
I was originally using replaceOccurencesOfString ""\" with "". But then it will do the following:
Hi, check out \this book \. Its cool
So, I concluded I need the above regular expression to match only " but not \" and then replace only those double quotes.
thanks
mbh

Answer 1

[^\\]\"
[^m] means does not match m

Answer 2

Not sure how this might translate to whatever is supported in the iOS apis, but, if they support anchoring (which I think all regex engines should), you're describing something like
(^|[^\])"
That is, match :
either the beginning of the string ^ or any character that's not
\ followed by:
the " character
If you want to do any sort of replacement, you'll have to grab the first (and only) group in the regex (that is the parenthetically grouped part of the expression) and use it in the replacement. Often this value labeled as $1 or \1 or something like that in your replacement string.
If the regex engine is PCRE based, of course you could put the grouped expression in a lookbehind so you wouldn't need to capture and save the capture in the replacement.

Answer 3

Not sure about regex, a simpler solution is,
NSString *str = #"\"Hi, check out \\\"this book \\\". Its cool\"";
NSLog(#"string before modification = %#", str);
str = [str stringByReplacingOccurrencesOfString:#"\\\"" withString:#"#$%$#"];
str = [str stringByReplacingOccurrencesOfString:#"\"" withString:#""];
str = [str stringByReplacingOccurrencesOfString:#"#$%$#" withString:#"\\\""];//assuming that the chances of having '#$%$#' in your string is zero, or else use more complicated word
NSLog(#"string after modification = %#", str);
Output:
string before modification = "Hi, check out \"this book \". Its cool"
string after modification = Hi, check out \"this book \". Its cool
Regex: [^\"].*[^\"]. which gives, Hi, check out \"this book \". Its cool

Answer 4

It looks like it's a JSON string? Perhaps created using json_encode() in PHP on the server? You should use the proper JSON parser in iOS. Don't use regex as you will run into bugs.
// fetch the data, eg this might return "Hi, check out \"this book \". Its cool"
NSData *data = [NSData dataWithContentsOfURL:#"http://example.com/foobar/"];
// decode the JSON string
NSError *error;
NSString *responseString = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&error];
// check if it worked or not
if (!responseString || ![responseString isKindOfClass:[NSString class]]) {
NSLog(#"failed to decode server response. error: %#", error);
return;
}
// print it out
NSLog(#"decoded response: %#", responseString);
The output will be:
Hi, check out "this book ". Its cool
Note: the JSON decoding API accepts an NSData object, not an NSString object. I'm assuming you also have a data object and are converting it to a string at some point... but if you're not, you can convert NSString to NSData using:
NSString *responseString = [NSJSONSerialization JSONObjectWithData:[myString dataUsingEncoding:NSUTF8StringEncoding] options:NSJSONReadingAllowFragments error:&error];
More details about JSON can be found at:
http://www.json.org
http://developer.apple.com/library/ios/#documentation/Foundation/Reference/NSJSONSerialization_Class/Reference/Reference.html