{$indexOfBytes: [<string>, <search value>, <start index - optional>, <end index - optional>]}
{$indexOfCP: [<string>, <search value>, <start index - optional>, <end index - optional>]}
{$indexOfArray: [<array>, <search value>, <start index - optional>, <end index - optional>]}

> db.coll.insert([
  {_id: 1, string: "hello world"}
]);
> db.coll.aggregate([{
  $project: {
    location: {$indexOfBytes: ["$string", "world"]}
  }
}]);
{_id: 1, location: 6}

// Example 2 - differentiating code points vs. bytes.
> db.coll.insert([
  {_id: 1, string: "øle"}
]);
> db.coll.aggregate([{
  $project: {
    byteLocation: {$indexOfBytes: ["$string", "le"]},
    cpLocation: {$indexOfCP: ["$string", "le"]}
  }
}]);
{_id: 1, byteLocation: 2, cpLocation: 1}

// Example 3 - using the start index.
> db.coll.insert([
  {_id: 1, string: "PREFIX|text with word FIX"},  // Contains "FIX", Should match.
  {_id: 2, string: "PREFIX|text without target"}  // Should not match.
]);
> db.coll.aggregate([{
  $project: {
    containsFix: {$indexOfCP: ["$string", "fix", {$strLenCP: "PREFIX|"}]}
  }
}]);
{_id: 1, containsFix: 22}
{_id: 2, containsFix: -1}


// Example 4 - using the start and end indices.
> db.coll.insert([
  {_id: 1, string: "PREFIX|text with word FIX|SUFFIX"},  // Contains "FIX", Should match.
  {_id: 2, string: "PREFIX|text without target|SUFFIX"}  // Should not match.
]);
> db.coll.aggregate([{
  $project: {
    containsFix: {
      $let: {
        vars: {
          startIndex: {$strLenCP: "PREFIX|"},  // 7
          endIndex: {$subtract: [0, {$strLenCP: "|SUFFIX"}]}  // -7
        },
        in: {$indexOfCP: ["$string", "fix", "$$startIndex", "$$endIndex"]}
      }
    }
  }
}]);
{_id: 1, containsFix: 22}
{_id: 2, containsFix: -1}