[[painless-walkthrough]]
=== A Brief Painless Walkthrough

To illustrate how Painless works, let's load some hockey stats into an Elasticsearch index:

[source,console]
----------------------------------------------------------------
PUT hockey/_bulk?refresh
{"index":{"_id":1}}
{"first":"johnny","last":"gaudreau","goals":[9,27,1],"assists":[17,46,0],"gp":[26,82,1],"born":"1993/08/13"}
{"index":{"_id":2}}
{"first":"sean","last":"monohan","goals":[7,54,26],"assists":[11,26,13],"gp":[26,82,82],"born":"1994/10/12"}
{"index":{"_id":3}}
{"first":"jiri","last":"hudler","goals":[5,34,36],"assists":[11,62,42],"gp":[24,80,79],"born":"1984/01/04"}
{"index":{"_id":4}}
{"first":"micheal","last":"frolik","goals":[4,6,15],"assists":[8,23,15],"gp":[26,82,82],"born":"1988/02/17"}
{"index":{"_id":5}}
{"first":"sam","last":"bennett","goals":[5,0,0],"assists":[8,1,0],"gp":[26,1,0],"born":"1996/06/20"}
{"index":{"_id":6}}
{"first":"dennis","last":"wideman","goals":[0,26,15],"assists":[11,30,24],"gp":[26,81,82],"born":"1983/03/20"}
{"index":{"_id":7}}
{"first":"david","last":"jones","goals":[7,19,5],"assists":[3,17,4],"gp":[26,45,34],"born":"1984/08/10"}
{"index":{"_id":8}}
{"first":"tj","last":"brodie","goals":[2,14,7],"assists":[8,42,30],"gp":[26,82,82],"born":"1990/06/07"}
{"index":{"_id":39}}
{"first":"mark","last":"giordano","goals":[6,30,15],"assists":[3,30,24],"gp":[26,60,63],"born":"1983/10/03"}
{"index":{"_id":10}}
{"first":"mikael","last":"backlund","goals":[3,15,13],"assists":[6,24,18],"gp":[26,82,82],"born":"1989/03/17"}
{"index":{"_id":11}}
{"first":"joe","last":"colborne","goals":[3,18,13],"assists":[6,20,24],"gp":[26,67,82],"born":"1990/01/30"}
----------------------------------------------------------------
// TESTSETUP

[discrete]
==== Accessing Doc Values from Painless

Document values can be accessed from a `Map` named `doc`.

For example, the following script calculates a player's total goals. This example uses a strongly typed `int` and a `for` loop.

[source,console]
----------------------------------------------------------------
GET hockey/_search
{
  "query": {
    "function_score": {
      "script_score": {
        "script": {
          "lang": "painless",
          "source": """
            int total = 0;
            for (int i = 0; i < doc['goals'].length; ++i) {
              total += doc['goals'][i];
            }
            return total;
          """
        }
      }
    }
  }
}
----------------------------------------------------------------

Alternatively, you could do the same thing using a script field instead of a function score:

[source,console]
----------------------------------------------------------------
GET hockey/_search
{
  "query": {
    "match_all": {}
  },
  "script_fields": {
    "total_goals": {
      "script": {
        "lang": "painless",
        "source": """
          int total = 0;
          for (int i = 0; i < doc['goals'].length; ++i) {
            total += doc['goals'][i];
          }
          return total;
        """
      }
    }
  }
}
----------------------------------------------------------------

The following example uses a Painless script to sort the players by their combined first and last names. The names are accessed using
`doc['first'].value` and `doc['last'].value`.

[source,console]
----------------------------------------------------------------
GET hockey/_search
{
  "query": {
    "match_all": {}
  },
  "sort": {
    "_script": {
      "type": "string",
      "order": "asc",
      "script": {
        "lang": "painless",
        "source": "doc['first.keyword'].value + ' ' + doc['last.keyword'].value"
      }
    }
  }
}
----------------------------------------------------------------


[discrete]
==== Missing values

`doc['field'].value` throws an exception if
the field is missing in a document.

To check if a document is missing a value, you can call
`doc['field'].size() == 0`.


[discrete]
==== Updating Fields with Painless

You can also easily update fields. You access the original source for a field as `ctx._source.<field-name>`.

First, let's look at the source data for a player by submitting the following request:

[source,console]
----------------------------------------------------------------
GET hockey/_search
{
  "query": {
    "term": {
      "_id": 1
    }
  }
}
----------------------------------------------------------------

To change player 1's last name to `hockey`, simply set `ctx._source.last` to the new value:

[source,console]
----------------------------------------------------------------
POST hockey/_update/1
{
  "script": {
    "lang": "painless",
    "source": "ctx._source.last = params.last",
    "params": {
      "last": "hockey"
    }
  }
}
----------------------------------------------------------------

You can also add fields to a document. For example, this script adds a new field that contains
the player's nickname,  _hockey_.

[source,console]
----------------------------------------------------------------
POST hockey/_update/1
{
  "script": {
    "lang": "painless",
    "source": """
      ctx._source.last = params.last;
      ctx._source.nick = params.nick
    """,
    "params": {
      "last": "gaudreau",
      "nick": "hockey"
    }
  }
}
----------------------------------------------------------------

[discrete]
[[modules-scripting-painless-dates]]
==== Dates

Date fields are exposed as
`ZonedDateTime`, so they support methods like `getYear`, `getDayOfWeek`
or e.g. getting milliseconds since epoch with `getMillis`. To use these
in a script, leave out the `get` prefix and continue with lowercasing the
rest of the method name. For example, the following returns every hockey
player's birth year:

[source,console]
----------------------------------------------------------------
GET hockey/_search
{
  "script_fields": {
    "birth_year": {
      "script": {
        "source": "doc.born.value.year"
      }
    }
  }
}
----------------------------------------------------------------

[discrete]
[[modules-scripting-painless-regex]]
==== Regular expressions

NOTE: Regexes are disabled by default because they circumvent Painless's
protection against long running and memory hungry scripts. To make matters
worse even innocuous looking regexes can have staggering performance and stack
depth behavior. They remain an amazing powerful tool but are too scary to enable
by default. To enable them yourself set `script.painless.regex.enabled: true` in
`elasticsearch.yml`. We'd like very much to have a safe alternative
implementation that can be enabled by default so check this space for later
developments!

Painless's native support for regular expressions has syntax constructs:

* `/pattern/`: Pattern literals create patterns. This is the only way to create
a pattern in painless. The pattern inside the ++/++'s are just
https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html[Java regular expressions].
See <<pattern-flags>> for more.
* `=~`: The find operator return a `boolean`, `true` if a subsequence of the
text matches, `false` otherwise.
* `==~`: The match operator returns a `boolean`, `true` if the text matches,
`false` if it doesn't.

Using the find operator (`=~`) you can update all hockey players with "b" in
their last name:

[source,console]
----------------------------------------------------------------
POST hockey/_update_by_query
{
  "script": {
    "lang": "painless",
    "source": """
      if (ctx._source.last =~ /b/) {
        ctx._source.last += "matched";
      } else {
        ctx.op = "noop";
      }
    """
  }
}
----------------------------------------------------------------

Using the match operator (`==~`) you can update all the hockey players whose
names start with a consonant and end with a vowel:

[source,console]
----------------------------------------------------------------
POST hockey/_update_by_query
{
  "script": {
    "lang": "painless",
    "source": """
      if (ctx._source.last ==~ /[^aeiou].*[aeiou]/) {
        ctx._source.last += "matched";
      } else {
        ctx.op = "noop";
      }
    """
  }
}
----------------------------------------------------------------

You can use the `Pattern.matcher` directly to get a `Matcher` instance and
remove all of the vowels in all of their last names:

[source,console]
----------------------------------------------------------------
POST hockey/_update_by_query
{
  "script": {
    "lang": "painless",
    "source": "ctx._source.last = /[aeiou]/.matcher(ctx._source.last).replaceAll('')"
  }
}
----------------------------------------------------------------

`Matcher.replaceAll` is just a call to Java's `Matcher`'s
https://docs.oracle.com/javase/8/docs/api/java/util/regex/Matcher.html#replaceAll-java.lang.String-[replaceAll]
method so it supports `$1` and `\1` for replacements:

[source,console]
----------------------------------------------------------------
POST hockey/_update_by_query
{
  "script": {
    "lang": "painless",
    "source": "ctx._source.last = /n([aeiou])/.matcher(ctx._source.last).replaceAll('$1')"
  }
}
----------------------------------------------------------------

If you need more control over replacements you can call `replaceAll` on a
`CharSequence` with a `Function<Matcher, String>` that builds the replacement.
This does not support `$1` or `\1` to access replacements because you already
have a reference to the matcher and can get them with `m.group(1)`.

IMPORTANT: Calling `Matcher.find` inside of the function that builds the
replacement is rude and will likely break the replacement process.

This will make all of the vowels in the hockey player's last names upper case:

[source,console]
----------------------------------------------------------------
POST hockey/_update_by_query
{
  "script": {
    "lang": "painless",
    "source": """
      ctx._source.last = ctx._source.last.replaceAll(/[aeiou]/, m ->
        m.group().toUpperCase(Locale.ROOT))
    """
  }
}
----------------------------------------------------------------

Or you can use the `CharSequence.replaceFirst` to make the first vowel in their
last names upper case:

[source,console]
----------------------------------------------------------------
POST hockey/_update_by_query
{
  "script": {
    "lang": "painless",
    "source": """
      ctx._source.last = ctx._source.last.replaceFirst(/[aeiou]/, m ->
        m.group().toUpperCase(Locale.ROOT))
    """
  }
}
----------------------------------------------------------------

Note: all of the `_update_by_query` examples above could really do with a
`query` to limit the data that they pull back. While you *could* use a
{ref}/query-dsl-script-query.html[script query] it wouldn't be as efficient
as using any other query because script queries aren't able to use the inverted
index to limit the documents that they have to check.