OPTIONS

$substr (aggregation)

$substr

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 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 is not encoding aware and if used improperly may produce a result string containing an invalid UTF-8 character sequence.

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

Example

Consider an inventory collection with the following documents:

{ "_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 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" }