[[analysis-pattern-capture-tokenfilter]]
=== Pattern Capture Token Filter

The `pattern_capture` token filter, unlike the `pattern` tokenizer,
emits a token for every capture group in the regular expression.
Patterns are not anchored to the beginning and end of the string, so
each pattern can match multiple times, and matches are allowed to
overlap.

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

The pattern capture token filter uses
http://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 http://www.regular-expressions.info/catastrophic.html[pathological regular expressions and how to avoid them].

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

For instance a pattern like :

[source,js]
--------------------------------------------------
"(([a-z]+)(\d*))"
--------------------------------------------------

when matched against:

[source,js]
--------------------------------------------------
"abc123def456"
--------------------------------------------------

would produce the tokens: [ `abc123`, `abc`, `123`, `def456`, `def`,
`456` ]

If `preserve_original` is set to `true` (the default) then it would also
emit the original token: `abc123def456`.

This is particularly useful for indexing text like camel-case code, eg
`stripHTML` where a user may search for `"strip html"` or `"striphtml"`:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/test/  -d '
{
   "settings" : {
      "analysis" : {
         "filter" : {
            "code" : {
               "type" : "pattern_capture",
               "preserve_original" : true,
               "patterns" : [
                  "(\\p{Ll}+|\\p{Lu}\\p{Ll}+|\\p{Lu}+)",
                  "(\\d+)"
               ]
            }
         },
         "analyzer" : {
            "code" : {
               "tokenizer" : "pattern",
               "filter" : [ "code", "lowercase" ]
            }
         }
      }
   }
}
'
--------------------------------------------------

When used to analyze the text

[source,js]
--------------------------------------------------
import static org.apache.commons.lang.StringEscapeUtils.escapeHtml
--------------------------------------------------

this emits the tokens: [ `import`, `static`, `org`, `apache`, `commons`,
`lang`, `stringescapeutils`, `string`, `escape`, `utils`, `escapehtml`,
`escape`, `html` ]

Another example is analyzing email addresses:

[source,js]
--------------------------------------------------
curl -XPUT localhost:9200/test/  -d '
{
   "settings" : {
      "analysis" : {
         "filter" : {
            "email" : {
               "type" : "pattern_capture",
               "preserve_original" : true,
               "patterns" : [
                  "([^@]+)",
                  "(\\p{L}+)",
                  "(\\d+)",
                  "@(.+)"
               ]
            }
         },
         "analyzer" : {
            "email" : {
               "tokenizer" : "uax_url_email",
               "filter" : [ "email", "lowercase",  "unique" ]
            }
         }
      }
   }
}
'
--------------------------------------------------

When the above analyzer is used on an email address like:

[source,js]
--------------------------------------------------
john-smith_123@foo-bar.com
--------------------------------------------------

it would produce the following tokens:

    john-smith_123@foo-bar.com, john-smith_123,
    john, smith, 123, foo-bar.com, foo, bar, com

Multiple patterns are required to allow overlapping captures, but also
means that patterns are less dense and easier to understand.

*Note:* All tokens are emitted in the same position, and with the same
character offsets, so when combined with highlighting, the whole
original token will be highlighted, not just the matching subset. For
instance, querying the above email address for `"smith"` would
highlight:

[source,js]
--------------------------------------------------
  <em>john-smith_123@foo-bar.com</em>
--------------------------------------------------

not:

[source,js]
--------------------------------------------------
  john-<em>smith</em>_123@foo-bar.com
--------------------------------------------------
