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Regular expressions are used for pattern matching, which is basically for findings strings within documents.

Sometimes when retrieving documents in a collection, you may not know exactly what the exact Field value to search for. Hence, one can use regular expressions to assist in retrieving data based on pattern matching search values.

In this tutorial, you will learn –

Using $regex operator for Pattern matching

The regex operator in MongoDB is used to search for specific strings in the collection. The following example shows how this can be done.

Let's assume that we have our same Employee collection which has the Field names of "Employeeid" and "EmployeeName". Let' also assume that we have the following documents in our collection.

Employee id Employee Name
22 NewMartin
2 Mohan
3 Joe
4 MohanR
100 Guru99
6 Gurang

Here in the below code we have used regex operator to specify the search criteria.

MongoDB Regular Expression (Regex) Tutorial

Code Explanation:

  1. Here we want to find all Employee Names which have the characters 'Gu' in it. Hence, we specify the $regex operator to define the search criteria of 'Gu'
  2. The printjson is being used to print each document which is returned by the query in a better way.

If the command is executed successfully, the following Output will be shown:

Output:

MongoDB Regular Expression (Regex) Tutorial

The output clearly shows that those documents wherein the Employee Name contains the 'Gu' characters are returned.

If suppose your collection has the following documents with an additional document which contained the Employee Name as "Guru999". If you entered the search criteria as "Guru99", it would also return the document which had "Guru999". But suppose if we didn't want this and only wanted to return the document with "Guru99". Then we can do this with exact pattern matching. To do an exact pattern matching, we will use the ^ and $ character. We will add the ^ character in the beginning of the string and $ at the end of the string.

Employee id Employee Name
22 NewMartin
2 Mohan
3 Joe
4 MohanR
100 Guru99
6 Gurang
8 Guru999

The following example shows how this can be done.

MongoDB Regular Expression (Regex) Tutorial

Code Explanation:

  1. Here in the search criteria, we are using the ^ and $ character. The ^ is used to make sure that the string starts with a certain character, and $ is used to ensure that the string ends with a certain character. So when the code executes it will fetch only the string with the name "Guru99".
  2. The printjson is being used to print each document which is returned by the query in a better way.

If the command is executed successfully, the following Output will be shown:

Output:

MongoDB Regular Expression (Regex) Tutorial

In the output, it is clearly visible that string "Guru99" is fetched.

Pattern Matching with $options

When using the regex operator one can also provide additional options by using the $options keyword. For example, suppose you wanted to find all the documents which had 'Gu' in their Employee Name, irrespective of whether it was case sensitive or insensitive. If such a result is desired, then we need to use the $options with case insensitivity parameter.

The following example shows how this can be done.

Let's assume that we have our same Employee collection which has the Field names of "Employeeid" and "EmployeeName".

Let' also assume that we have the following documents in our collection.

Employee id Employee Name
22 NewMartin
2 Mohan
3 Joe
4 MohanR
100 Guru99
6 Gurang
7 GURU99

Now if we run the same query as in the last topic, we would never see the document with "GURU99" in the result. To ensure this comes in the result set, we need to add the $options "I" parameter.

MongoDB Regular Expression (Regex) Tutorial

Code Explanation:

  1. The $options with 'I' parameter (which means case insensitivity) specifies that we want to carry out the search no matter if we find the letters 'Gu' in lower or upper case.

If the command is executed successfully, the following Output will be shown:

Output:

MongoDB Regular Expression (Regex) Tutorial

  1. The output clearly shows that even though one document has the upper case 'Gu' , the document still gets displayed in the result set.

Pattern matching without the regex operator

One can also do pattern matching without the regex operator. The following example shows how this can be done.

MongoDB Regular Expression (Regex) Tutorial

Code Explanation:

  1. The "//" options basically means to specify your search criteria within these delimiters. Hence, we are specifying /Gu/ to again find those documents which have 'Gu' in their EmployeeName.

If the command is executed successfully, the following Output will be shown:

Output:

MongoDB Regular Expression (Regex) Tutorial

The output clearly shows that those documents wherein the Employee Name contains the 'Gu' characters are returned.

Fetching last 'n' documents from a collection

There are various ways to get the last n documents in a collection.

Let's look at one of the ways via the following steps

The following example shows how this can be done.

Let's assume that we have our same Employee collection which has the Field names of "Employeeid" and "EmployeeName".

Let' also assume that we have the following documents in our collection:

Employee id Employee Name
22 NewMartin
2 Mohan
3 Joe
4 MohanR
100 Guru99
6 Gurang
7 GURU99

MongoDB Regular Expression (Regex) Tutorial

Code Explanation:

1) When querying for the documents, use the sort function to sort the records in reverse order based on the _id field value in the collection. The -1 basically indicates to sort the documents in reverse order or descending order so that the last document becomes the first document to be displayed.

2) Then use the limit clause to just display the number of records you want. Here we have set the limit clause (2), so it will fetch the last two documents.

If the command is executed successfully, the following Output will be shown:

Output:

MongoDB Regular Expression (Regex) Tutorial

The output clearly shows that the last two documents in the collection are displayed. Hence we have clearly shown that to fetch the last 'n' documents in the collection, we can first sort the documents in descending order and then use the limit clause to return the 'n' number of documents which are required.

Note: If the search is performed on a string which is greater than say 38,000 characters, it will not display the right results.

Summary:

  • Pattern matching can be achieved by the $regex operator. This operator can be used to find for certain strings in the collection.
  • The ^ and $ symbol can be used for exact text searches with ^ being used to make sure that the string starts with a certain character and $ used to ensure that the string ends with a certain character.
  • The 'i' along with the $regex operator can be used to specify case insensitivity so that strings can be searched whether they are in lower case or upper case.
  • The delimiters // can also be used for pattern matching.
  • Use a combination of sort and the limit function to return the last n documents in the collection. The sort function can be used to return the documents in descending order after which the limit clause can be used to limit the number of documents being returned.

 

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