$substr (expression operator)

Definition

$substr

Deprecated since version 3.4: $substr is now an alias for $substrBytes.

Returns a substring of a string, starting at a specified index position and including the specified number of characters. The index is zero-based.

$substr has the following syntax:

{ $substr: [ <string>, <start>, <length> ] }

The arguments can be any valid expression as long as the first argument resolves to a string, and the second and third arguments resolve to integers. For more information on expressions, see Expressions.

Behavior

If <start> is a negative number, $substr returns an empty string "".

If <length> is a negative number, $substr returns a substring that starts at the specified index and includes the rest of the string.

$substr only has a well-defined behavior for strings of ASCII characters.

Example

Consider an inventory collection with the following documents:

db.inventory.insertMany( [
   { _id: 1, item: "ABC1", quarter: "13Q1", description: "product 1" },
   { _id: 2, item: "ABC2", quarter: "13Q4", description: "product 2" },
   { _id: 3, item: "XYZ1", quarter: "14Q2", description: null }
] )

The following operation uses the $substr operator to separate the quarter value into a yearSubstring and a quarterSubstring:

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            yearSubstring: { $substr: [ "$quarter", 0, 2 ] },
            quarterSubtring: { $substr: [ "$quarter", 2, -1 ] }
          }
      }
   ]
)

The operation returns the following results:

{ _id: 1, item: "ABC1", yearSubstring: "13", quarterSubtring: "Q1" }
{ _id: 2, item: "ABC2", yearSubstring: "13", quarterSubtring: "Q4" }
{ _id: 3, item: "XYZ1", yearSubstring: "14", quarterSubtring: "Q2" }