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: