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Tidying Up Input Text

Tokenizers produce the best results when the input text is clean, valid text, where valid means that it follows the punctuation rules that the Unicode algorithm expects. Quite often, though, the text we need to process is anything but clean. Cleaning it up before tokenization improves the quality of the output.

Tokenizing HTML

Passing HTML through the standard tokenizer or the icu_tokenizer produces poor results. These tokenizers just don’t know what to do with the HTML tags. For example:

GET /_analyzer?tokenizer=standard
<p>Some d&eacute;j&agrave; vu <a href="http://somedomain.com>">website</a>

The standard tokenizer confuses HTML tags and entities, and emits the following tokens: p, Some, d, eacute, j, agrave, vu, a, href, http, somedomain.com, website, a. Clearly not what was intended!

Character filters can be added to an analyzer to preprocess the text before it is passed to the tokenizer. In this case, we can use the html_strip character filter to remove HTML tags and to decode HTML entities such as é into the corresponding Unicode characters.

Character filters can be tested out via the analyze API by specifying them in the query string:

GET /_analyzer?tokenizer=standard&char_filters=html_strip
<p>Some d&eacute;j&agrave; vu <a href="http://somedomain.com>">website</a>

To use them as part of the analyzer, they should be added to a custom analyzer definition:

PUT /my_index
{
    "settings": {
        "analysis": {
            "analyzer": {
                "my_html_analyzer": {
                    "tokenizer":     "standard",
                    "char_filter": [ "html_strip" ]
                }
            }
        }
    }
}

Once created, our new my_html_analyzer can be tested with the analyze API:

GET /my_index/_analyzer?analyzer=my_html_analyzer
<p>Some d&eacute;j&agrave; vu <a href="http://somedomain.com>">website</a>

This emits the tokens that we expect: Some, déjà, vu, website.

Tidying Up Punctuation

The standard tokenizer and icu_tokenizer both understand that an apostrophe within a word should be treated as part of the word, while single quotes that surround a word should not. Tokenizing the text You’re my 'favorite'. would correctly emit the tokens You’re, my, favorite.

Unfortunately, Unicode lists a few characters that are sometimes used as apostrophes:

U+0027

Apostrophe (')—the original ASCII character

U+2018

Left single-quotation mark (‘)—opening quote when single-quoting

U+2019

Right single-quotation mark (’)—closing quote when single-quoting, but also the preferred character to use as an apostrophe

Both tokenizers treat these three characters as an apostrophe (and thus as part of the word) when they appear within a word. Then there are another three apostrophe-like characters:

U+201B

Single high-reversed-9 quotation mark (‛)—same as U+2018 but differs in appearance

U+0091

Left single-quotation mark in ISO-8859-1—should not be used in Unicode

U+0092

Right single-quotation mark in ISO-8859-1—should not be used in Unicode

Both tokenizers treat these three characters as word boundaries—​a place to break text into tokens. Unfortunately, some publishers use U+201B as a stylized way to write names like M‛coy, and the second two characters may well be produced by your word processor, depending on its age.

Even when using the `acceptable'' quotation marks, a word written with a single right quotation mark—`You’re—is not the same as the word written with an apostrophe—`You’re`—which means that a query for one variant will not find the other.

Fortunately, it is possible to sort out this mess with the mapping character filter, which allows us to replace all instances of one character with another. In this case, we will replace all apostrophe variants with the simple U+0027 apostrophe:

PUT /my_index
{
  "settings": {
    "analysis": {
      "char_filter": { (1)
        "quotes": {
          "type": "mapping",
          "mappings": [ (2)
            "\\u0091=>\\u0027",
            "\\u0092=>\\u0027",
            "\\u2018=>\\u0027",
            "\\u2019=>\\u0027",
            "\\u201B=>\\u0027"
          ]
        }
      },
      "analyzer": {
        "quotes_analyzer": {
          "tokenizer":     "standard",
          "char_filter": [ "quotes" ] (3)
        }
      }
    }
  }
}

  1. We define a custom char_filter called quotes that maps all apostrophe variants to a simple apostrophe.

  2. For clarity, we have used the JSON Unicode escape syntax for each character, but we could just have used the characters themselves: "‘⇒'".

  3. We use our custom quotes character filter to create a new analyzer called quotes_analyzer.

As always, we test the analyzer after creating it:

GET /my_index/_analyze?analyzer=quotes_analyzer
You’re my ‘favorite’ M‛Coy

This example returns the following tokens, with all of the in-word quotation marks replaced by apostrophes: You’re, my, favorite, M’Coy.

The more effort that you put into ensuring that the tokenizer receives good-quality input, the better your search results will be.