NSPredicate predicateWithFormat not evaluating format specifier - objective-c

Question

I have a (to me) curious case with NSPredicate's predicateWithFormat: method.
Using the following I log the description of two NSPredicate instances to the console:
NSNumber *myNumber = [NSNumber numberWithInt:1];
NSString *predicateFormatByHand = [NSString stringWithFormat:#"self MATCHES 'chp%#_img[0-9]+\\.png'", myNumber];
NSPredicate *firstPredicate = [NSPredicate predicateWithFormat:predicateFormatByHand];
NSLog(#"firstPredicate description: %#", firstPredicate);
NSPredicate *secondPredicate = [NSPredicate predicateWithFormat:#"self MATCHES 'chp%#_img[0-9]+\\.png'", myNumber];
NSLog(#"secondPredicate description: %#", secondPredicate);
This outputs:
firstPredicate description: SELF MATCHES "chp1_img[0-9]+.png"
secondPredicate description: SELF MATCHES "chp%#_img[0-9]+.png"
I would expect these descriptions to be the same.
Can someone explain why they are not?
(Following this question I've played with various escape sequences for the embedded single-quotes but when doing so keep having NSPredicate complain that it cannot then parse the format string. I'd be grateful to know what's going on.)
UPDATE: one answer suggested it's an issue with using NSNumber rather than an int, so:
NSPredicate *thirdPredicate = [NSPredicate predicateWithFormat:#"self MATCHES 'chp%d_img[0-9]+\\.png'", [myNumber intValue]];
NSLog(#"thirdPredicate description: %#", thirdPredicate);
I began with this originally, but alas the output is the same:
thirdPredicate description: SELF MATCHES "chp%d_img[0-9]+.png"
(Something means the format specifier is not evaluated.)

Answer 1

The answer is simple: the parser used by NSPredicate assumes that anything inside the quote marks is a string literal, and does not attempt to do any substitutions on its contents. It you need to have a dynamic string value, you will have to build the string before substituting it into the predicate format string, as in your first example.

Answer 2

...because the predicate is not such thing than string.
for any of the predicates you should use two format specifier 100% safety only:
one for the key (%K); and
one for the value (%#);
you cannot format neither the key nor the value when you add them to the predicate. this is why your second (and third) predicates are not formatted inside the value.
you can format the value before you add it to the predicate like:
NSNumber *myNumber = [NSNumber numberWithInt:1];
NSString *string = [NSString stringWithFormat:#"chp%#_img[0-9]+\\.png", myNumber];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"SELF MATCHES %#", string];
NSLog(#"%#", predicate);
the result is:
SELF MATCHES "chp1_img[0-9]+\\.png"
...and never forget my first sentence: the predicates and the strings are not the same thing.

Answer 3

My interpretation of
%# is a var arg substitution for an object value
in the "Predicate Programming Guide" is that %# can only be used for substituting a value that a Core Data object can be compared against. For example
NSNumber *myNumber = [NSNumber numberWithInt:1];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"count = %#", myNumber];
is perfectly valid if "count" is a Number attribute of the entity. It is similar to binding values to SQLite prepared statements.
If %# could be used for general string formatting in predicates, then there would be no need to have two different format specifiers %K and %# for key paths and values.