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$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

The following example uses the movies collection from the sample_mflix sample dataset. For details on how to load this dataset into your deployment, see Load the sample dataset.

This aggregation operation uses the $substr operator to separate the title value into a titleStart and a titleRest. The second expression uses a negative <length> to return the characters from index 3 to the end of the string:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $project:
        {
          _id: 0,
          title: 1,
          titleStart: { $substr: [ "$title", 0, 3 ] },
          titleRest: { $substr: [ "$title", 3, -1 ] }
        }
   }
] )

[
  {
    title: 'Centennial',
    titleStart: 'Cen',
    titleRest: 'tennial'
  },
  {
    title: 'Baseball',
    titleStart: 'Bas',
    titleRest: 'eball'
  }
]

Back

$strLenCP

$substrBytes