Normalizers

Normalizers are similar to analyzers except that they may only emit a single token. As a consequence, they do not have a tokenizer and only accept a subset of the available char filters and token filters. Only the filters that work on a per-character basis are allowed. For instance a lowercasing filter would be allowed, but not a stemming filter, which needs to look at the keyword as a whole. The current list of filters that can be used in a normalizer is following: arabic_normalization, asciifolding, bengali_normalization, cjk_width, decimal_digit, elision, german_normalization, hindi_normalization, indic_normalization, lowercase, persian_normalization, scandinavian_folding, serbian_normalization, sorani_normalization, uppercase.

Elasticsearch ships with a lowercase built-in normalizer. For other forms of normalization a custom configuration is required.

Custom normalizers

Custom normalizers take a list of character filters and a list of token filters.

  1. PUT index
  2. {
  3.   "settings": {
  4.     "analysis": {
  5.       "char_filter": {
  6.         "quote": {
  7.           "type": "mapping",
  8.           "mappings": [
  9.             "« => \"",
  10.             "» => \""
  11.           ]
  12.         }
  13.       },
  14.       "normalizer": {
  15.         "my_normalizer": {
  16.           "type": "custom",
  17.           "char_filter": ["quote"],
  18.           "filter": ["lowercase", "asciifolding"]
  19.         }
  20.       }
  21.     }
  22.   },
  23.   "mappings": {
  24.     "properties": {
  25.       "foo": {
  26.         "type": "keyword",
  27.         "normalizer": "my_normalizer"
  28.       }
  29.     }
  30.   }
  31. }