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Elastic Docs ›Elasticsearch Guide [5.2] ›Analysis ›Tokenizers

Pattern Tokenizer

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Pattern Tokenizer

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The pattern tokenizer uses a regular expression to either split text into terms whenever it matches a word separator, or to capture matching text as terms.

The default pattern is \W+, which splits text whenever it encounters non-word characters.

Beware of Pathological Regular Expressions

The pattern tokenizer uses Java Regular Expressions.

A badly written regular expression could run very slowly or even throw a StackOverflowError and cause the node it is running on to exit suddenly.

Read more about pathological regular expressions and how to avoid them.

Example output

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POST _analyze
{
  "tokenizer": "pattern",
  "text": "The foo_bar_size's default is 5."
}

The above sentence would produce the following terms:

[ The, foo_bar_size, s, default, is, 5 ]

Configuration

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The pattern tokenizer accepts the following parameters:

pattern

A Java regular expression, defaults to \W+.

flags

Java regular expression flags. lags should be pipe-separated, eg "CASE_INSENSITIVE|COMMENTS".

group

Which capture group to extract as tokens. Defaults to -1 (split).

Example configuration

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In this example, we configure the pattern tokenizer to break text into tokens when it encounters commas:

PUT my_index
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "my_tokenizer"
        }
      },
      "tokenizer": {
        "my_tokenizer": {
          "type": "pattern",
          "pattern": ","
        }
      }
    }
  }
}

POST my_index/_analyze
{
  "analyzer": "my_analyzer",
  "text": "comma,separated,values"
}

The above example produces the following terms:

[ comma, separated, values ]

In the next example, we configure the pattern tokenizer to capture values enclosed in double quotes (ignoring embedded escaped quotes \"). The regex itself looks like this:

"((?:\\"|[^"]|\\")*)"

And reads as follows:

  • A literal "

  • Start capturing:

    • A literal \" OR any character except "
    • Repeat until no more characters match
  • A literal closing "

When the pattern is specified in JSON, the " and \ characters need to be escaped, so the pattern ends up looking like:

\"((?:\\\\\"|[^\"]|\\\\\")+)\"
PUT my_index
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "my_tokenizer"
        }
      },
      "tokenizer": {
        "my_tokenizer": {
          "type": "pattern",
          "pattern": "\"((?:\\\\\"|[^\"]|\\\\\")+)\"",
          "group": 1
        }
      }
    }
  }
}

POST my_index/_analyze
{
  "analyzer": "my_analyzer",
  "text": "\"value\", \"value with embedded \\\" quote\""
}

The above example produces the following two terms:

[ value, value with embedded \" quote ]
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