String objects are not normalized unless you perform that step manually. This means that comparing strings that contain combining character sequences can potentially lead to wrong results. Both isEqual: and isEqualToString: compare strings byte for byte. If you want precomposed and decomposed variants to match, you must normalize the strings first.

nsstring-unicode-comparison.m

// http://www.objc.io/issue-9/unicode.html
NSString *s = @"\u00E9"; // é
NSString *t = @"e\u0301"; // e + ´
BOOL isEqual = [s isEqualToString:t];
NSLog(@"%@ is %@ to %@", s, isEqual ? @"equal" : @"not equal", t);
// => é is not equal to é

// Normalizing to form C
NSString *sNorm = [s precomposedStringWithCanonicalMapping];
NSString *tNorm = [t precomposedStringWithCanonicalMapping];
BOOL isEqualNorm = [sNorm isEqualToString:tNorm];
NSLog(@"%@ is %@ to %@", sNorm, isEqualNorm ? @"equal" : @"not equal", tNorm);
// => é is equal to é