MongoDB Evaluation Query operator - $regex

Description

A regular expression is an important way of specifying a pattern for a complex search. The MongoDB $regex operator provides such a way to search a string after matching a specified pattern.

In MongoDB, matching the pattern can do by using regular expression objects or using the $regex operator.

Our database name is 'myinfo' and our collection name is 'student'. Here, is the collection bellow.

Sample collection "student"

[
        {
                "f_name" : "Zenny",
                "sex" : "Female",
                "class" : "VI",
                "age" : 12,
                "grd_point" : 32.6342
        },
        {
                "f_name" : "Paul",
                "sex" : "Male",
                "class" : "VII",
                "age" : 13,
                "grd_point" : 29.5904
        },
        {
                "f_name" : "Tom",
                "sex" : "Male",
                "class" : "VI",
                "age" : 11,
                "grd_point" : 30.1257
        },
        {
                "f_name" : "Lassy",
                "sex" : "Female",
                "class" : "VIII",
                "age" : 13,
                "grd_point" : 28.2514
        },
        {
                "f_name" : "Peter",
                "sex" : "Male",
                "class" : "VI",
                "age" : 11,
                "grd_point" : 31.5201
        }
]	
		

Example of MongoDB Evaluation Query operator - $regex

If we want to select all documents from the collection "student" which satisfying the condition -

The f_name must start with letter "P"

the following mongodb command can be used :

>db.student.find( { f_name: { $regex: 'P.*'} } ).pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

SQL equivalent command is

SELECT * 
FROM student 
WHERE f_name LIKE 'P%';

Output:

{
        "_id" : ObjectId("527b3cc65ceafed9b2254a95"),
        "f_name" : "Paul",
        "sex" : "Male",
        "class" : "VII",
        "age" : 13,
        "grd_point" : 29.5904
}
{
        "_id" : ObjectId("527b3cc65ceafed9b2254a98"),
        "f_name" : "Peter",
        "sex" : "Male",
        "class" : "VI",
        "age" : 11,
        "grd_point" : 31.5201
}

If we write the above mongodb statement like below no output will be available.

>db.student.find( { f_name: { $regex: 'p.*'} } ).pretty();

Because no f_name is starting with letter 'p'. In that case to avoid the case sensetivity we can write the above statement in following manner. Here is the example -

>db.student.find( { f_name: { $regex: 'p.*', $options: 'i' } } ).pretty();

The option "$option : i" is used to avoid the case sensitivity.

The above statement can be wirtten without using $regex but using regular expression objects. Here are the commands -

>db.student.find( { f_name: /p/i } ).pretty();
>db.student.find( { f_name: /P/ } ).pretty();
>db.student.find( { f_name: /^p/i } ).pretty();

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