Add $findChar or $indexOf operator for strings to find position of specific character (or substring)

{$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}