$indexOfBytes (expression operator)

Definition

$indexOfBytes

Searches a string for an occurrence of a substring and returns the UTF-8 byte index (zero-based) of the first occurrence. If the substring is not found, returns -1.

$indexOfBytes has the following operator expression syntax:

{ $indexOfBytes: [ <string expression>, <substring expression>, <start>, <end> ] }

Operand	Description
<string expression>	Can be any valid expression as long as it resolves to a string. For more information on expressions, see Expressions. If the string expression resolves to a value of null or refers to a field that is missing, $indexOfBytes returns null. If the string expression does not resolve to a string or null nor refers to a missing field, $indexOfBytes returns an error.
<substring expression>	Can be any valid expression as long as it resolves to a string. For more information on expressions, see Expressions.
<start>	Optional An integral number that specifies the starting index position for the search. Can be any valid expression that resolves to a non-negative integral number.
<end>	Optional An integral number that specifies the ending index position for the search. Can be any valid expression that resolves to a non-negative integral number. If you specify a <end> index value, you should also specify a <start> index value; otherwise, $indexOfBytes uses the <end> value as the <start> index value instead of the <end> value.

Behavior

• If <string expression> is null, $indexOfBytes returns null.
• If $indexOfBytes is called on a field that doesn't exist in the document, $indexOfBytes returns null.
• If <string expression> is not a string and not null, $indexOfBytes returns an error.
• If <substring expression> is null, $indexOfBytes returns an error.
• If <start> or <end> is a negative number, $indexOfBytes returns an error.
• If <start> is a number greater than <end>, $indexOfBytes returns -1.
• If <start> is a number greater than the byte length of the string, $indexOfBytes returns -1.
• If <start> or <end> is given a value that is not an integer, $indexOfBytes returns an error.
• If the <substring expression> is found multiple times within the <string expression>, then $indexOfBytes returns the index of the first <substring expression> found.

Some short examples to highlight different behavior:

Examples

Consider an inventory collection with the following documents:

db.inventory.insertMany( [
   { _id: 1, item: "foo" },
   { _id: 2, item: "fóofoo" },
   { _id: 3, item: "the foo bar" },
   { _id: 4, item: "hello world fóo" },
   { _id: 5, item: null },
   { _id: 6, amount: 3 }
] )

The following operation uses the $indexOfBytes operator to retrieve the indexes at which the string foo is located in each item:

db.inventory.aggregate(
   [
     {
       $project:
          {
            byteLocation: { $indexOfBytes: [ "$item", "foo" ] },
          }
      }
   ]
)

The operation returns the following results:

{ _id: 1, byteLocation: "0" }
{ _id: 2, byteLocation: "4" }
{ _id: 3, byteLocation: "4" }
{ _id: 4, byteLocation: "-1" }
{ _id: 5, byteLocation: null }
{ _id: 6, byteLocation: null }