{
  "name": "substr",
  "type": "function",
  "description": "Returns a part of a string.",
  "arguments": [ {"arg":"input_string","description":"the full input string"},
  {"arg":"startpos","description":"integer representing start position to extract from"},
  {"arg":"length","description":"integer representing length of string to extract"}
  ],
  "examples": [ { "expression":"substr('HELLO WORLD',3,5)", "returns":"'LLO W'"}]
}