Humans are nothing if not inventive, and human language reflects that. Changing the case of a word seems like such a simple task, until you have to deal with multiple languages.

Take, for example, the lowercase German letter ß. Converting that to upper case gives you SS, which converted back to lowercase gives you ss. Or consider the Greek letter ς (sigma, when used at the end of a word). Converting it to uppercase results in Σ, which converted back to lowercase, gives you σ.

The whole point of lowercasing terms is to make them more likely to match, not less! In Unicode, this job is done by case folding rather than by lowercasing. Case folding is the act of converting words into a (usually lowercase) form that does not necessarily result in the correct spelling, but does allow case-insensitive comparisons.

For instance, the letter ß, which is already lowercase, is folded to ss. Similarly, the lowercase ς is folded to σ, to make σ, ς, and Σ comparable, no matter where the letter appears in a word.

The default normalization form that the icu_normalizer token filter uses is nfkc_cf. Like the nfkc form, this does the following:

But it also does this:

In other words, nfkc_cf is the equivalent of the lowercase token filter, but suitable for use with all languages. The on-steroids equivalent of the standard analyzer would be the following:

PUT /my_index
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_lowercaser": {
          "tokenizer": "icu_tokenizer",
          "filter":  [ "icu_normalizer" ] 
        }
      }
    }
  }
}

We can compare the results of running Weißkopfseeadler and WEISSKOPFSEEADLER (the uppercase equivalent) through the standard analyzer and through our Unicode-aware analyzer:

GET /_analyze?analyzer=standard 
Weißkopfseeadler WEISSKOPFSEEADLER

GET /my_index/_analyze?analyzer=my_lowercaser 
Weißkopfseeadler WEISSKOPFSEEADLER

The standard analyzer emits two different, incomparable tokens, while our custom analyzer produces tokens that are comparable, regardless of the original case.