Painless supports all of Java’s control flow statements except the switch statement.

Painless also supports the for in syntax from Groovy:

for (item : list) {
  ...
}

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

boolean isNegative(def x) { x < 0 }
...
if (isNegative(someVar)) {
  ...
}

Lambda expressions and method references work the same as in Java.

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).

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.

Pattern p = /[aeiou]/