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Aggregation Pipeline Operators

$regexMatch (aggregation)

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  • Definition
  • Syntax
  • Behavior
  • Examples

Definition

$regexMatch

New in version 4.2.

Performs a regular expression (regex) pattern matching and returns:

  • true if a match exists.

  • false if a match doesn't exist.

MongoDB uses Perl compatible regular expressions (i.e. "PCRE" ) version 8.41 with UTF-8 support.

Prior to MongoDB 4.2, aggregation pipeline can only use the query operator $regex in the $match stage. For more information on using regex in a query, see $regex.

Syntax

The $regexMatch operator has the following syntax:

{ $regexMatch: { input: <expression> , regex: <expression>, options: <expression> } }
Field
Description

The string on which you wish to apply the regex pattern. Can be a string or any valid expression that resolves to a string.

The regex pattern to apply. Can be any valid expression that resolves to either a string or regex pattern /<pattern>/. When using the regex /<pattern>/, you can also specify the regex options i and m (but not the s or x options):

  • "pattern"

  • /<pattern>/

  • /<pattern>/<options>

Alternatively, you can also specify the regex options with the options field. To specify the s or x options, you must use the options field.

You cannot specify options in both the regex and the options field.

Optional. The following <options> are available for use with regular expression.

Note

You cannot specify options in both the regex and the options field.

Option
Description
i
Case insensitivity to match both upper and lower cases. You can specify the option in the options field or as part of the regex field.
m

For patterns that include anchors (i.e. ^ for the start, $ for the end), match at the beginning or end of each line for strings with multiline values. Without this option, these anchors match at beginning or end of the string.

If the pattern contains no anchors or if the string value has no newline characters (e.g. \n), the m option has no effect.

x

"Extended" capability to ignore all white space characters in the pattern unless escaped or included in a character class.

Additionally, it ignores characters in-between and including an un-escaped hash/pound (#) character and the next new line, so that you may include comments in complicated patterns. This only applies to data characters; white space characters may never appear within special character sequences in a pattern.

The x option does not affect the handling of the VT character (i.e. code 11).

You can specify the option only in the options field.

s

Allows the dot character (i.e. .) to match all characters including newline characters.

You can specify the option only in the options field.

Returns

The operator returns a boolean:

  • true if a match exists.

  • false if a match doesn't exist.

Tip

See also:

Behavior

$regexMatch and Collation

$regexMatch ignores the collation specified for the collection, db.collection.aggregate(), and the index, if used.

For example, the create a sample collection with collation strength 1 (i.e. compare base character only and ignore other differences such as case and diacritics):

db.createCollection( "myColl", { collation: { locale: "fr", strength: 1 } } )

Insert the following documents:

db.myColl.insertMany([
   { _id: 1, category: "café" },
   { _id: 2, category: "cafe" },
   { _id: 3, category: "cafE" }
])

Using the collection's collation, the following operation performs a case-insensitive and diacritic-insensitive match:

db.myColl.aggregate( [ { $match: { category: "cafe" } } ] )

The operation returns the following 3 documents:

{ "_id" : 1, "category" : "café" }
{ "_id" : 2, "category" : "cafe" }
{ "_id" : 3, "category" : "cafE" }

However, the aggregation expression $regexMatch ignores collation; that is, the following regular expression pattern matching examples are case-sensitive and diacritic sensitive:

db.myColl.aggregate( [ { $addFields: { results: { $regexMatch: { input: "$category", regex: /cafe/ }  } } } ] )
db.myColl.aggregate(
   [ { $addFields: { results: { $regexMatch: { input: "$category", regex: /cafe/ }  } } } ],
   { collation: { locale: "fr", strength: 1 } }       // Ignored in the $regexMatch
)

Both operations return the following:

{ "_id" : 1, "category" : "café", "results" : false }
{ "_id" : 2, "category" : "cafe", "results" : true }
{ "_id" : 3, "category" : "cafE", "results" : false }

To perform a case-insensitive regex pattern matching, use the i Option instead. See i Option for an example.

Examples

$regexMatch and Its Options

To illustrate the behavior of the $regexMatch operator as discussed in this example, create a sample collection products with the following documents:

db.products.insertMany([
   { _id: 1, description: "Single LINE description." },
   { _id: 2, description: "First lines\nsecond line" },
   { _id: 3, description: "Many spaces before     line" },
   { _id: 4, description: "Multiple\nline descriptions" },
   { _id: 5, description: "anchors, links and hyperlinks" },
   { _id: 6, description: "métier work vocation" }
])

By default, $regexMatch performs a case-sensitive match. For example, the following aggregation performs a case-sensitive $regexMatch on the description field. The regex pattern /line/ does not specify any grouping:

db.products.aggregate([
   { $addFields: { result: { $regexMatch: { input: "$description", regex: /line/ } } } }
])

The operation returns the following:

{ "_id" : 1, "description" : "Single LINE description.", "result" : false }
{ "_id" : 2, "description" : "First lines\nsecond line", "result" : true }
{ "_id" : 3, "description" : "Many spaces before     line", "result" : true }
{ "_id" : 4, "description" : "Multiple\nline descriptions", "result" : true }
{ "_id" : 5, "description" : "anchors, links and hyperlinks", "result" : false }
{ "_id" : 6, "description" : "métier work vocation", "result" : false }

The following regex pattern /lin(e|k)/ specifies a grouping (e|k) in the pattern:

db.products.aggregate([
   { $addFields: { result: { $regexMatch: { input: "$description", regex: /lin(e|k)/ } } } }
])

The operation returns the following:

{ "_id" : 1, "description" : "Single LINE description.", "result" : false }
{ "_id" : 2, "description" : "First lines\nsecond line", "result" : true }
{ "_id" : 3, "description" : "Many spaces before     line", "result" : true }
{ "_id" : 4, "description" : "Multiple\nline descriptions", "result" : true }
{ "_id" : 5, "description" : "anchors, links and hyperlinks", "result" : true }
{ "_id" : 6, "description" : "métier work vocation", "result" : false }

i Option

Note

You cannot specify options in both the regex and the options field.

To perform case-insensitive pattern matching, include the i option as part of the regex field or in the options field:

// Specify i as part of the regex field
{ $regexMatch: { input: "$description", regex: /line/i } }
// Specify i in the options field
{ $regexMatch: { input: "$description", regex: /line/, options: "i" } }
{ $regexMatch: { input: "$description", regex: "line", options: "i" } }

For example, the following aggregation performs a case-insensitive $regexMatch on the description field. The regex pattern /line/ does not specify any grouping:

db.products.aggregate([
   { $addFields: { result: { $regexMatch: { input: "$description", regex: /line/i } } } }
])

The operation returns the following documents:

{ "_id" : 1, "description" : "Single LINE description.", "result" : true }
{ "_id" : 2, "description" : "First lines\nsecond line", "result" : true }
{ "_id" : 3, "description" : "Many spaces before     line", "result" : true }
{ "_id" : 4, "description" : "Multiple\nline descriptions", "result" : true }
{ "_id" : 5, "description" : "anchors, links and hyperlinks", "result" : false }
{ "_id" : 6, "description" : "métier work vocation", "result" : false }

m Option

Note

You cannot specify options in both the regex and the options field.

To match the specified anchors (e.g. ^, $) for each line of a multiline string, include the m option as part of the regex field or in the options field:

// Specify m as part of the regex field
{ $regexMatch: { input: "$description", regex: /line/m } }
// Specify m in the options field
{ $regexMatch: { input: "$description", regex: /line/, options: "m" } }
{ $regexMatch: { input: "$description", regex: "line", options: "m" } }

The following example includes both the i and the m options to match lines starting with either the letter s or S for multiline strings:

db.products.aggregate([
   { $addFields: { result: { $regexMatch: { input: "$description", regex: /^s/im } } } }
])

The operation returns the following:

{ "_id" : 1, "description" : "Single LINE description.", "result" : true }
{ "_id" : 2, "description" : "First lines\nsecond line", "result" : true }
{ "_id" : 3, "description" : "Many spaces before     line", "result" : false }
{ "_id" : 4, "description" : "Multiple\nline descriptions", "result" : false }
{ "_id" : 5, "description" : "anchors, links and hyperlinks", "result" : false }
{ "_id" : 6, "description" : "métier work vocation", "result" : false }

x Option

Note

You cannot specify options in both the regex and the options field.

To ignore all unescaped white space characters and comments (denoted by the un-escaped hash # character and the next new-line character) in the pattern, include the s option in the options field:

// Specify x in the options field
{ $regexMatch: { input: "$description", regex: /line/, options: "x" } }
{ $regexMatch: { input: "$description", regex: "line", options: "x" } }

The following example includes the x option to skip unescaped white spaces and comments:

db.products.aggregate([
   { $addFields: { returns: { $regexMatch: { input: "$description", regex: /lin(e|k) # matches line or link/, options:"x" } } } }
])

The operation returns the following:

{ "_id" : 1, "description" : "Single LINE description.", "returns" : false }
{ "_id" : 2, "description" : "First lines\nsecond line", "returns" : true }
{ "_id" : 3, "description" : "Many spaces before     line", "returns" : true }
{ "_id" : 4, "description" : "Multiple\nline descriptions", "returns" : true }
{ "_id" : 5, "description" : "anchors, links and hyperlinks", "returns" : true }
{ "_id" : 6, "description" : "métier work vocation", "returns" : false }

s Option

Note

You cannot specify options in both the regex and the options field.

To allow the dot character (i.e. .) in the pattern to match all characters including the new line character, include the s option in the options field:

// Specify s in the options field
{ $regexMatch: { input: "$description", regex: /m.*line/, options: "s" } }
{ $regexMatch: { input: "$description", regex: "m.*line", options: "s" } }

The following example includes the s option to allow the dot character (i.e. .) to match all characters including new line as well as the i option to perform a case-insensitive match:

db.products.aggregate([
   { $addFields: { returns: { $regexMatch: { input: "$description", regex:/m.*line/, options: "si"  } } } }
])

The operation returns the following:

{ "_id" : 1, "description" : "Single LINE description.", "returns" : false }
{ "_id" : 2, "description" : "First lines\nsecond line", "returns" : false }
{ "_id" : 3, "description" : "Many spaces before     line", "returns" : true }
{ "_id" : 4, "description" : "Multiple\nline descriptions", "returns" : true }
{ "_id" : 5, "description" : "anchors, links and hyperlinks", "returns" : false }
{ "_id" : 6, "description" : "métier work vocation", "returns" : false }

Use $regexMatch to Check Email Address

Create a sample collection feedback with the following documents:

db.feedback.insertMany([
   { "_id" : 1, comment: "Hi, I'm just reading about MongoDB -- aunt.arc.tica@example.com"  },
   { "_id" : 2, comment: "I wanted to concatenate a string" },
   { "_id" : 3, comment: "How do I convert a date to string? Contact me at either cam@mongodb.com or c.dia@mongodb.com" },
   { "_id" : 4, comment: "It's just me. I'm testing.  fred@MongoDB.com" }
])

The following aggregation uses the $regexMatch to check if the comment field contains an email address with @mongodb.com and categorize the feedback as Employee or External.

db.feedback.aggregate( [
    { $addFields: {
       "category": { $cond: { if:  { $regexMatch: { input: "$comment", regex: /[a-z0-9_.+-]+@mongodb.com/i } },
                              then: "Employee",
                              else: "External" } }
    } },

The operation returns the following documents:

{ "_id" : 1, "comment" : "Hi, I'm just reading about MongoDB -- aunt.arc.tica@example.com", "category" : "External" }
{ "_id" : 2, "comment" : "I wanted to concatenate a string", "category" : "External" }
{ "_id" : 3, "comment" : "How do I convert a date to string? Contact me at either cam@mongodb.com or c.dia@mongodb.com", "category" : "Employee" }
{ "_id" : 4, "comment" : "It's just me. I'm testing.  fred@MongoDB.com", "category" : "Employee" }

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