length()
The length() function takes one argument of the JSON pointer type and returns the length of the passed value. For example, length(/message) returns a length of 10 when a key message exists in the event and has a value of 1234567890.
Example
{
  "event": {
    "/message": "1234567890"
  },
  "expression": "length(/message)",
  "expected_output": 10
}