[[analysis-pattern-replace-charfilter]]
=== Pattern replace character filter
++++
<titleabbrev>Pattern replace</titleabbrev>
++++

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.

[WARNING]
.Beware of Pathological Regular Expressions
========================================

The pattern replace character filter uses
https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html[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 https://www.regular-expressions.info/catastrophic.html[pathological regular expressions and how to avoid them].

========================================

[discrete]
=== Configuration

The `pattern_replace` character filter accepts the following parameters:

[horizontal]
`pattern`::

    A https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html[Java regular expression]. Required.

`replacement`::

    The replacement string, which can reference capture groups using the
    `$1`..`$9` syntax, as explained
    https://docs.oracle.com/javase/8/docs/api/java/util/regex/Matcher.html#appendReplacement-java.lang.StringBuffer-java.lang.String-[here].

`flags`::

    Java regular expression https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html#field.summary[flags].
    Flags should be pipe-separated, eg `"CASE_INSENSITIVE|COMMENTS"`.

[discrete]
=== Example configuration

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`:

[source,console]
----------------------------
PUT my-index-00001
{
  "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-00001/_analyze
{
  "analyzer": "my_analyzer",
  "text": "My credit card is 123-456-789"
}
----------------------------
// TEST[s/\$1//]
// the test framework doesn't like the $1 so we just throw it away rather than
// try to get it to work properly. At least we are still testing the charfilter.

The above example produces the following terms:

[source,text]
---------------------------
[ 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:

[source,console]
----------------------------
PUT my-index-00001
{
  "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-00001/_analyze
{
  "analyzer": "my_analyzer",
  "text": "The fooBarBaz method"
}
----------------------------

/////////////////////

[source,console-result]
----------------------------
{
  "tokens": [
    {
      "token": "the",
      "start_offset": 0,
      "end_offset": 3,
      "type": "<ALPHANUM>",
      "position": 0
    },
    {
      "token": "foo",
      "start_offset": 4,
      "end_offset": 6,
      "type": "<ALPHANUM>",
      "position": 1
    },
    {
      "token": "bar",
      "start_offset": 7,
      "end_offset": 9,
      "type": "<ALPHANUM>",
      "position": 2
    },
    {
      "token": "baz",
      "start_offset": 10,
      "end_offset": 13,
      "type": "<ALPHANUM>",
      "position": 3
    },
    {
      "token": "method",
      "start_offset": 14,
      "end_offset": 20,
      "type": "<ALPHANUM>",
      "position": 4
    }
  ]
}
----------------------------

/////////////////////

The above returns the following terms:

[source,text]
----------------------------
[ 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:

[source,console]
----------------------------
PUT my-index-00001/_doc/1?refresh
{
  "text": "The fooBarBaz method"
}

GET my-index-00001/_search
{
  "query": {
    "match": {
      "text": "bar"
    }
  },
  "highlight": {
    "fields": {
      "text": {}
    }
  }
}
----------------------------
// TEST[continued]

The output from the above is:

[source,console-result]
----------------------------
{
  "timed_out": false,
  "took": $body.took,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped" : 0,
    "failed": 0
  },
  "hits": {
    "total" : {
        "value": 1,
        "relation": "eq"
    },
    "max_score": 0.2876821,
    "hits": [
      {
        "_index": "my-index-00001",
        "_type": "_doc",
        "_id": "1",
        "_score": 0.2876821,
        "_source": {
          "text": "The fooBarBaz method"
        },
        "highlight": {
          "text": [
            "The foo<em>Ba</em>rBaz method" <1>
          ]
        }
      }
    ]
  }
}
----------------------------
// TESTRESPONSE[s/"took".*/"took": "$body.took",/]

<1> Note the incorrect highlight.
