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Evaluation query operators

Evaluation query operators return data based on the evaluation of a specified expression.

OperatorDescription
$modMatches documents where the field element is divided by a given value and returns a the specified remainder value
$regexMatches documents where a field matches a specified regular expression query

For the examples in this section, insert the following documents into the catalog collection:

db.catalog.insertMany([
{
product: 'bottle',
price: 15,
stock: 1
},
{
product: 'spoon',
price: 500,
stock: 0
},
{
product: 'cup',
price: 100,
stock: 14
},
{
product: 'BoWL',
price: 56,
stock: 5
},
{
product: 'boTtLe',
price: 20,
stock: 3
}
])

$mod

Syntax: { <field>: { $mod: [ <divisor-value>, <modulus> ] } }

The $mod operator matches documents where the field element divided by a given value returns a specified remainder (otherwise known as a modulus). The mathematical operation for this is field-value % divisor-value = modulus.

Example: The following query returns all the documents where the value of the "stock" field is evenly divisible by 2:

db.catalog.find({
stock: {
$mod: [2, 0]
}
})

The output:

[
{
_id: ObjectId("63e3ac0184f488929a3f737a"),
product: 'spoon',
price: 500,
stock: 0
},
{
_id: ObjectId("63e3ac0184f488929a3f737b"),
product: 'cup',
price: 100,
stock: 14
}
]
caution

Note that the $mod expression returns an error if you only have a single element in the array, more than two elements in the array, or if the array is empty. It also rounds down decimal input down to zero (e.g. $mod: [ 3.5 , 2 ] is executed as $mod: [ 3 , 2 ]).

$regex

Syntax: { <field>: { $regex: '<expression-string>', $options: '<flag>' } }

Other syntaxes: { <field>: { $regex: /<expression-string>/, $options: '<flag>' } } and { <field>: /<expression-string>/<flag> }.

To use regular expression for queries on particular fields, use the $regex operator.

Example: The following query returns all the documents where the value of the "product" field starts with the letter "b":

db.catalog.find({
product: {
$regex: /^b/
}
})

The output:

[
{
_id: ObjectId("63e4ce469695494b86bf2b2d"),
product: 'bottle',
price: 15,
stock: 1
},
{
_id: ObjectId("63e4ce469695494b86bf2b31"),
product: 'boTtLe',
price: 20,
stock: 3
}
]

$options is an optional parameter that specifies the regular expression flags to use, such as:

  • Case-insensitivity (i)
  • Multi-line matching (m)
  • Dot character matching (s)
note

The regex flag for ignoring white spaces (x) is not currently supported. Follow here for more updates.

To perform case-insensitive matching, use the i flag in the regex expression.

Example: The following query returns all the documents where the value of the "product" field is equal to "bottle" (case-insensitive):

db.catalog.find({
product: {
$regex: /bottle/i
}
})

The output:

[
{
_id: ObjectId("63e3ac0184f488929a3f7379"),
product: 'bottle',
price: 15,
stock: 1
},
{
_id: ObjectId("63e3ac0184f488929a3f737d"),
product: 'boTtLe',
price: 20,
stock: 3
}
]