[[painless-syntax]]
=== Painless Syntax

experimental[The Painless scripting language is new and is still marked as experimental. The syntax or API may be changed in the future in non-backwards compatible ways if required.]

[float]
[[control-flow]]
==== Control flow

Painless supports all of Java's https://docs.oracle.com/javase/tutorial/java/nutsandbolts/flow.html[
control flow statements] except the `switch` statement.

Painless also supports the `for in` syntax from Groovy:

[source,painless]
---------------------------------------------------------
for (item : list) {
  ...
}
---------------------------------------------------------

[float]
[[functions]]
==== Functions

You can declare functions at the beginning of a Painless script, for example:

[source,painless]
---------------------------------------------------------
boolean isNegative(def x) { x < 0 }
...
if (isNegative(someVar)) {
  ...
}
---------------------------------------------------------

[float]
[[lambda-expressions]]
==== Lambda expressions
Lambda expressions and method references work the same as in https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html[Java].

[source,painless]
---------------------------------------------------------
list.removeIf(item -> item == 2);
list.removeIf((int item) -> item == 2);
list.removeIf((int item) -> { item == 2 });
list.sort((x, y) -> x - y);
list.sort(Integer::compare);
---------------------------------------------------------

You can make method references to functions within the script with `this`,
for example `list.sort(this::mycompare)`.

[float]
[[patterns]]
==== Patterns

Regular expression constants are directly supported. To ensure fast performance,
this is the only mechanism for creating patterns. Regular expressions
are always constants and compiled efficiently a single time.

[source,painless]
---------------------------------------------------------
Pattern p = /[aeiou]/
---------------------------------------------------------

[float]
[[pattern-flags]]
===== Pattern flags

You can define flags on patterns in Painless by adding characters after the
trailing `/` like `/foo/i` or `/foo \w #comment/iUx`. Painless exposes all of
the flags from Java's
https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html[
Pattern class] using these characters:

[cols="<,<,<",options="header",]
|=======================================================================
| Character | Java Constant | Example
|`c` | CANON_EQ                  | `'å' ==~ /å/c` (open in hex editor to see)
|`i` | CASE_INSENSITIVE          | `'A' ==~ /a/i`
|`l` | LITERAL                   | `'[a]' ==~ /[a]/l`
|`m` | MULTILINE                 | `'a\nb\nc' =~ /^b$/m`
|`s` | DOTALL (aka single line)  | `'a\nb\nc' =~ /.b./s`
|`U` | UNICODE_CHARACTER_CLASS   | `'Ɛ' ==~ /\\w/U`
|`u` | UNICODE_CASE              | `'Ɛ' ==~ /ɛ/iu`
|`x` | COMMENTS (aka extended)   | `'a' ==~ /a #comment/x`
|=======================================================================

[float]
[[painless-deref]]
==== Dereferences

Like lots of languages, Painless uses `.` to reference fields and call methods:

[source,painless]
---------------------------------------------------------
String foo = 'foo';
TypeWithGetterOrPublicField bar = new TypeWithGetterOrPublicField()
return foo.length() + bar.x
---------------------------------------------------------

Like Groovy, Painless uses `?.` to perform null-safe references, with the
result being `null` if the left hand side is `null`:

[source,painless]
---------------------------------------------------------
String foo = null;
return foo?.length()  // Returns null
---------------------------------------------------------

Unlike Groovy, Painless doesn't support writing to `null` values with this
operator:

[source,painless]
---------------------------------------------------------
TypeWithSetterOrPublicField foo = null;
foo?.x = 'bar'  // Compile error
---------------------------------------------------------