elasticsearch/docs/reference/text-analysis/analysis-pattern-replace-charfilter.md

---

navigation_title: "Pattern replace" mapped_pages:

• https://www.elastic.co/guide/en/elasticsearch/reference/current/analysis-pattern-replace-charfilter.html ---

Pattern replace character filter [analysis-pattern-replace-charfilter]

The pattern_replace character filter uses a regular expression to match characters which should be replaced with the specified replacement string. The replacement string can refer to capture groups in the regular expression.

::::{admonition} Beware of Pathological Regular Expressions :class: warning

The pattern replace character filter 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.

::::

Configuration [_configuration_23]

The pattern_replace character filter accepts the following parameters:

pattern : A Java regular expression. Required.

replacement : The replacement string, which can reference capture groups using the $1..$9 syntax, as explained here.

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

Example configuration [_example_configuration_15]

In this example, we configure the pattern_replace character filter to replace any embedded dashes in numbers with underscores, i.e 123-456-789 → 123_456_789:

PUT my-index-000001
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "standard",
          "char_filter": [
            "my_char_filter"
          ]
        }
      },
      "char_filter": {
        "my_char_filter": {
          "type": "pattern_replace",
          "pattern": "(\\d+)-(?=\\d)",
          "replacement": "$1_"
        }
      }
    }
  }
}

POST my-index-000001/_analyze
{
  "analyzer": "my_analyzer",
  "text": "My credit card is 123-456-789"
}

The above example produces the following terms:

[ My, credit, card, is, 123_456_789 ]

::::{warning} Using a replacement string that changes the length of the original text will work for search purposes, but will result in incorrect highlighting, as can be seen in the following example. ::::

This example inserts a space whenever it encounters a lower-case letter followed by an upper-case letter (i.e. fooBarBaz → foo Bar Baz), allowing camelCase words to be queried individually:

PUT my-index-000001
{
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "standard",
          "char_filter": [
            "my_char_filter"
          ],
          "filter": [
            "lowercase"
          ]
        }
      },
      "char_filter": {
        "my_char_filter": {
          "type": "pattern_replace",
          "pattern": "(?<=\\p{Lower})(?=\\p{Upper})",
          "replacement": " "
        }
      }
    }
  },
  "mappings": {
    "properties": {
      "text": {
        "type": "text",
        "analyzer": "my_analyzer"
      }
    }
  }
}

POST my-index-000001/_analyze
{
  "analyzer": "my_analyzer",
  "text": "The fooBarBaz method"
}

The above returns the following terms:

[ the, foo, bar, baz, method ]

Querying for bar will find the document correctly, but highlighting on the result will produce incorrect highlights, because our character filter changed the length of the original text:

PUT my-index-000001/_doc/1?refresh
{
  "text": "The fooBarBaz method"
}

GET my-index-000001/_search
{
  "query": {
    "match": {
      "text": "bar"
    }
  },
  "highlight": {
    "fields": {
      "text": {}
    }
  }
}

The output from the above is:

{
  "timed_out": false,
  "took": $body.took,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped" : 0,
    "failed": 0
  },
  "hits": {
    "total" : {
        "value": 1,
        "relation": "eq"
    },
    "max_score": 0.2876821,
    "hits": [
      {
        "_index": "my-index-000001",
        "_id": "1",
        "_score": 0.2876821,
        "_source": {
          "text": "The fooBarBaz method"
        },
        "highlight": {
          "text": [
            "The foo<em>Ba</em>rBaz method" <1>
          ]
        }
      }
    ]
  }
}

1. Note the incorrect highlight.