Decompose ligatures (preferably with ICU)

Question

I'm looking for a clean way to decompose ligatures (e.g. œ -> oe) in a Unicode string. Is there a way to do this without enumerating all the rules one by one? Something like this rule for removing diacritical marks but for ligatures instead.

Answer 1

Reading the ICU documentation on transforms, it seems you need this one

Latin-ASCII

So I suppose passing doing something similar as your linked question should work (code is untested):

#include <unicode/utypes.h>
#include <unicode/unistr.h>
#include <unicode/translit.h>

std::string asciiifyUTF8(const std::string& str)
{
    // UTF-8 std::string -> UTF-16 UnicodeString
    UnicodeString source = UnicodeString::fromUTF8(StringPiece(str));

    // Transliterate UTF-16 UnicodeString
    UErrorCode status = U_ZERO_ERROR;
    Transliterator *asciiConverter = Transliterator::createInstance(
        "Latin-ASCII", UTRANS_FORWARD, status);
    asciiConverter->transliterate(source);
    // TODO: handle errors with status

    // UTF-16 UnicodeString -> UTF-8 std::string
    std::string result;
    source.toUTF8String(result);

    return result;
}

Note this will also strip accents and the like, as the output will be limited to ASCII characters. You can filter the input the transform applies to, but the (French) ligature characters aren't in any special region (there are really only two in French)? Browsing through the Unicode characters, there seem to be a whole lot more ligatures in various other scripts, so the above suggestion might not be what you are looking for.