What is normalized UTF-8 all about?

If two unicode strings are canonically equivalent the strings are really the same, only using different unicode sequences. For example Ä can be represented either using the character Ä or a combination of A and ◌̈.

If the strings are only compatibility equivalent the strings aren't necessarily the same, but they may be the same in some contexts. E.g. ff could be considered same as ff.

So, if you are comparing strings you should use canonical equivalence, because compatibility equivalence isn't real equivalence.

But if you want to sort a set of strings it might make sense to use compatibility equivalence as the are nearly identical.

Everything You Never Wanted to Know about Unicode Normalization

Canonical Normalization

Unicode includes multiple ways to encode some characters, most notably accented characters. Canonical normalization changes the code points into a canonical encoding form. The resulting code points should appear identical to the original ones barring any bugs in the fonts or rendering engine.

When To Use

Because the results appear identical, it is always safe to apply canonical normalization to a string before storing or displaying it, as long as you can tolerate the result not being bit for bit identical to the input.

Canonical normalization comes in 2 forms: NFD and NFC. The two are equivalent in the sense that one can convert between these two forms without loss. Comparing two strings under NFC will always give the same result as comparing them under NFD.

NFD

NFD has the characters fully expanded out. This is the faster normalization form to calculate, but the results in more code points (i.e. uses more space).

If you just want to compare two strings that are not already normalized, this is the preferred normalization form unless you know you need compatibility normalization.

NFC

NFC recombines code points when possible after running the NFD algorithm. This takes a little longer, but results in shorter strings.

Compatibility Normalization

Unicode also includes many characters that really do not belong, but were used in legacy character sets. Unicode added these to allow text in those character sets to be processed as Unicode, and then be converted back without loss.

Compatibility normalization converts these to the corresponding sequence of "real" characters, and also performs canonical normalization. The results of compatibility normalization may not appear identical to the originals.

Characters that include formatting information are replaced with ones that do not. For example the character ⁹ gets converted to 9. Others don't involve formatting differences. For example the roman numeral character Ⅸ is converted to the regular letters IX.

Obviously, once this transformation has been performed, it is no longer possible to losslessly convert back to the original character set.

When to use

The Unicode Consortium suggests thinking of compatibility normalization like a ToUpperCase transform. It is something that may be useful in some circumstances, but you should not just apply it willy-nilly.

An excellent use case would be a search engine since you would probably want a search for 9 to match ⁹.

One thing you should probably not do is display the result of applying compatibility normalization to the user.

NFKC/NFKD

Compatibility normalization form comes in two forms NFKD and NFKC. They have the same relationship as between NFD and C.

Any string in NFKC is inherently also in NFC, and the same for the NFKD and NFD. Thus NFKD(x)=NFD(NFKC(x)), and NFKC(x)=NFC(NFKD(x)), etc.

Conclusion

If in doubt, go with canonical normalization. Choose NFC or NFD based on the space/speed trade-off applicable, or based on what is required by something you are inter-operating with.

Normal forms (of Unicode, not databases) deal primarily (exclusively?) with characters that have diacritical marks. Unicode provides some characters with "built in" diacritical marks, such as U+00C0, "Latin Capital A with Grave". The same character can be created from a `Latin Capital A" (U+0041) with a "Combining Grave Accent" (U+0300). That means even though the two sequences produce the same resulting character, a byte-by-byte comparison will show them as being completely different.

Normalization is an attempt at dealing with that. Normalizing assures (or at least tries to) that all the characters are encoded the same way -- either all using a separate combining diacritical mark where needed, or all using a single code point wherever possible. From a viewpoint of comparison, it doesn't really matter a whole lot which you choose -- pretty much any normalized string will compare properly with another normalized string.

In this case, "compatibility" means compatibility with code that assumes that one code point equals one character. If you have code like that, you probably want to use the compatibility normal form. Although I've never seen it stated directly, the names of the normal forms imply that the Unicode consortium considers it preferable to use separate combining diacritical marks. This requires more intelligence to count the actual characters in a string (as well as things like breaking a string intelligently), but is more versatile.

If you're making full use of ICU, chances are that you want to use the canonical normal form. If you're trying to write code on your own that (for example) assumes a code point equals a character, then you probably want the compatibility normal form that makes that true as often as possible.

Some characters, for example a letter with an accent (say, é) can be represented in two ways - a single code point U+00E9 or the plain letter followed by a combining accent mark U+0065 U+0301. Ordinary normalization will choose one of these to always represent it (the single code point for NFC, the combining form for NFD).

For characters that could be represented by multiple sequences of base characters and combining marks (say, "s, dot below, dot above" vs putting dot above then dot below or using a base character that already has one of the dots), NFD will also pick one of these (below goes first, as it happens)

The compatibility decompositions include a number of characters that "shouldn't really" be characters but are because they were used in legacy encodings. Ordinary normalization won't unify these (to preserve round-trip integrity - this isn't an issue for the combining forms because no legacy encoding [except a handful of vietnamese encodings] used both), but compatibility normalization will. Think like the "kg" kilogram sign that appears in some East Asian encodings (or the halfwidth/fullwidth katakana and alphabet), or the "fi" ligature in MacRoman.

See http://unicode.org/reports/tr15/ for more details.