## Groups & Capturing

Grouping is the most useful feature of regex. Grouping can be done by placing regular expression inside round brackets. In this article, we will see how to extract and replace data using groups.

It unifies the regular expressions inside it as a single unit. Let's look at its usages one by one:

1. It makes the regular expression more readable and sometimes it is an inevitable thing.

   Suppose, we want to match both the sentences in the above text, then grouping is the inevitable thing.

2. To apply quantifiers to one or more expressions.

   Similarly, you can use other quantifiers.

3. To extract and replace substrings using groups. So, we call groups Capturing groups, becuase we are capturing data(substrings) using groups.

   Data Extraction

   Observe the code below.

       var str = "2020-01-20";

       // Pattern string
       var pattern = /(\d{4})-(\d{2})-(\d{2})/g;

          //            ^       ^       ^
          //group-no:   1       2       3

       var result = pattern.exec(str);

       // printing
       console.log(result);

       // Data extraction
       console.log(result[1]); // First group
       console.log(result[2]); // Second group
       console.log(result[3]); // Third group

   The output will be:

       [
         '2020-01-20',
         '2020',
         '01',
         '20',
         index: 0,
         input: '2020-01-20',
         groups: undefined
       ]
       2020
       01
       20

   In the output array, the first data is a match string followed by the matched groups in the order.

   Data Replacement

   `Replace` is another function, which can be used to replace and rearrange the data using regex. Observe the code below.

       var str = "2020-01-20";

       // Pattern string
       var pattern = /(\d{4})-(\d{2})-(\d{2})/g;

          //            ^       ^       ^
          //group-no:   1       2       3

       // Data replacement using $group_no
       var ans=str.replace(pattern, '$3-$2-$1');

       console.log(ans);
       // Output will be: 20-01-2020

   As you can see, we have used `$group_no` to indicate the capturing group.

## Problems

Predict the output of the following regex:

1. RegEx: `([abc]){2,}(one|two)`
   Text: `aone cqtwo abone actwo abcbtwoone abbcccone`

   Answer:

2. RegEx: `([\dab]+(r|c)){2}`
   Text: `1r2c ar4ccc 12abr12abc acac, accaca, acaaca aaar1234234c, aaa1234234c 194brar, 134bcbb-c`

   Answer: