Is there a way to use NSString stringByFoldingWithOptions to unfold the single French 'œ' character into 'oe'?

Question

For a diacritics-agnostic full text search feature, I use the following code to convert accented characters like é or Ö into their lowercase non-accented form e and o

[[inputString stringByFoldingWithOptions: 
    NSCaseInsensitiveSearch
    + NSDiacriticInsensitiveSearch
    + NSWidthInsensitiveSearch
locale: [NSLocale currentLocale]] lowercaseString];

This works. However, I found no way to convert special characters whose base form consists of multiple characters like the French œ (as in "sœur") or the German ß (as in 'Fluß'). I would like to convert them into oe and ss respectively. I found no flag for stringByFoldingWithOptions and did not find anything on the web.

EDIT

ß is actually handled correctly by the above code. It converts to ss.

Answer 1

From worst to best solution.

Solution 1 will work only for æ and ß and fails for everything else (œ, ij, ff, fi, fl, ffi, ffl, ſt, st, ...):

NSString *result = [[[NSString alloc] initWithData:[inputString dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES] encoding:NSASCIIStringEncoding] autorelease];

Solution 2 will work for most ligatures and only fails for æ, œ and ij. I've tried all possible NSLocale, so it's not the issue here:

NSString *result = [inputString stringByFoldingWithOptions:NSCaseInsensitiveSearch | NSDiacriticInsensitiveSearch | NSWidthInsensitiveSearch locale:[NSLocale currentLocale]];

Solution 3 will work for most ligatures and only fails for œ:

NSString *result = [[[NSString alloc] initWithData:[[inputString precomposedStringWithCompatibilityMapping] dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES] encoding:NSASCIIStringEncoding] autorelease];

Which means œ will always need to be manually handled. And best solution is to combine either solution 2 or 3 with a manual string replacement.

Solution 2bis:

inputString = [inputString stringByReplacingOccurrencesOfString:@"æ" withString:@"ae" options:NSCaseInsensitiveSearch range:NSMakeRange(0, [inputString length])];
inputString = [inputString stringByReplacingOccurrencesOfString:@"œ" withString:@"oe" options:NSCaseInsensitiveSearch range:NSMakeRange(0, [inputString length])];
inputString = [inputString stringByReplacingOccurrencesOfString:@"ij" withString:@"ij" options:NSCaseInsensitiveSearch range:NSMakeRange(0, [inputString length])];
NSString *result = [inputString stringByFoldingWithOptions:NSCaseInsensitiveSearch | NSDiacriticInsensitiveSearch | NSWidthInsensitiveSearch locale:[NSLocale currentLocale]];

Solution 3bis:

inputString = [inputString stringByReplacingOccurrencesOfString:@"Œ" withString:@"OE"];
inputString = [inputString stringByReplacingOccurrencesOfString:@"œ" withString:@"oe"];
NSString *result = [[[NSString alloc] initWithData:[[inputString precomposedStringWithCompatibilityMapping] dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES] encoding:NSASCIIStringEncoding] autorelease];

Knowing I might be missing some replacements with solution 2bis and NSLocale is unpredictable, best solution is 3bis. And also this last solution allows you to keep case sensitivity if you need.