CJK width token filter

The cjk_width token filter normalizes Chinese, Japanese, and Korean (CJK) tokens by converting full-width ASCII characters to their standard (half-width) ASCII equivalents and half-width katakana characters to their full-width equivalents.

Converting full-width ASCII characters

In CJK texts, ASCII characters (such as letters and numbers) can appear in full-width form, occupying the space of two half-width characters. Full-width ASCII characters are typically used in East Asian typography for alignment with the width of CJK characters. However, for the purposes of indexing and searching, these full-width characters need to be normalized to their standard (half-width) ASCII equivalents.

The following example illustrates ASCII character normalization:

        Full-Width:              ＡＢＣＤＥ １２３４５
        Normalized (half-width): ABCDE 12345

Converting half-width katakana characters

The cjk_width token filter converts half-width katakana characters to their full-width counterparts, which are the standard form used in Japanese text. This normalization, illustrated in the following example, is important for consistency in text processing and searching:

        Half-Width katakana:               ｶﾀｶﾅ
        Normalized (full-width) katakana:  カタカナ

Example

The following example request creates a new index named cjk_width_example_index and defines an analyzer with the cjk_width filter:

PUT /cjk_width_example_index
{
  "settings": {
    "analysis": {
      "analyzer": {
        "cjk_width_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": ["cjk_width"]
        }
      }
    }
  }
}

Generated tokens

Use the following request to examine the tokens generated using the analyzer:

POST /cjk_width_example_index/_analyze
{
  "analyzer": "cjk_width_analyzer",
  "text": "Ｔｏｋｙｏ 2024 ｶﾀｶﾅ"
}

The response contains the generated tokens:

{
  "tokens": [
    {
      "token": "Tokyo",
      "start_offset": 0,
      "end_offset": 5,
      "type": "<ALPHANUM>",
      "position": 0
    },
    {
      "token": "2024",
      "start_offset": 6,
      "end_offset": 10,
      "type": "<NUM>",
      "position": 1
    },
    {
      "token": "カタカナ",
      "start_offset": 11,
      "end_offset": 15,
      "type": "<KATAKANA>",
      "position": 2
    }
  ]
}