“Capturing groups” are parts of RegEx which are used to group one or more characters inside a RegEx. Let me try to explain with a simple example.

Email address as Source Text : abc123@qsys400.com

Here is a simple RegEx to match this email address : [a-zA-Z]\w+@\w+\.\w+

This RegEx matches the given Email address perfectly. If we look closely, we can break this email address in following parts-

1. User name “abc123” : is a group of alphanumeric characters starting with English alphabet. =>  [a-zA-Z]\w+
2. address sign “@” : is a string literal to create a partition between user name and domain name. => @
3. Domain name “qsys400” : is a group of alphanumeric characters. => \w+
4. Dot “.” : String literal dot to create a partition between domain and top level domain name. => \.
5. Top level domain “com”: is a group of alphanumeric characters. => \w+

So if we write the above RegEx again to group character as described above, it will look as ([a-zA-Z]\w+)(@)(\w+)(\.)(\w+)

Now this RegEx has 5 groups

1.  ([a-zA-Z]\w+)
2. (@)
3. (\w+)
4. (\.)
5. (\w+)

What is the use of these groups?

Every programming language which supports RegEx, gives functionality to get values corresponding to each  groups in RegEx.

For example let say I want to get user name and domain name (including top level domain) from the email address

RegEx :  ([a-zA-Z]\w+)@(\w+\.\w+)

In this RegEx there are following 2 groups

1. ([a-zA-Z]\w+) : from User name (like abc123)
2. (\w+\.\w+) :  for domain name (like qsys400.com)

“@” is not a part of any of the groups.

So when a programming language processes this RegEx, it gives functionality to get(or capture) the value for GROUP 1 and GROUP2. Where value of GROUP1 will be user name from the email and value of GROUP2 will be domain name .

Capturing Groups Numbering

Each capturing group, in a RegEx, gets a unique number starting from 1. It is very simple. From left, start giving number to each opening parenthesis “(“-

(A)(B) ===> Group 1 (A) ===> Group 2  (B)

Same rule applies for Nested capturing groups-

(A(B))(C) ==> Group 1 (A(B)) ==> Group 2 (B) ==> Group 3 (C)

Example:

RegEx  (\w{3})  : It creates a group of 3 characters

Source Text : “abcdef ghi”

Based on RegEx [without capturing groups] \w{3} will find 3 matches

1. abc
2. def
3. ghi

With “Capturing groups”, RegEx  (\w{3}) will create one group for each match

Group ZERO

1.  Group numbering starts with number 1.
2. In some RegEx engines, there is a GROUP ZERO which contains the value of complete RegEx match.

Example

RegEx (\d{3})\w+

Source Tex : “123abc%def”

1. This complete RegEx will match “123abc” from source text i.e. GROUP ZERO = “123abc”
   1. With in this match Group 1 will contains the value “123”

 Numbering with Quantifiers

If a quantifier is added with any group in RegEx , number of groups in RegEx does not change which means “quantifiers do not impact number of groups”.

(A){3} ==> It says that there must be exact 3 occurrences of capturing group (A)

But number of groups in this RegEx is still one. So, you will get value of GROUP 1 (there is no GROUP 2 or GROUP 3). With every new match, previous value of the group will be overridden by new value.

Example

RegEx  (\w{3})+  : Last character “+” adds the quantifier (ONE or MORE occurrences) to the RegEx group with creates a group of 3 characters

Source Text : “abcdef ghi”

1. Due to quantifier on First Match, this RegEx will consume “abcdef” (due to space before “ghi”)
   1. Text “abcdef” has 2 sets of length 3 characters
      1. “abc”
      2. “def”
   2. So technically First match contains 2 groups.
   3. But there is only 1 group defined in this RegEx i.e (\w{3})
   4. Here is how it works
      1. After consuming “abcdef” ,RegEx will try to find group value (it has only 1 Group)
         • First match at “abc” ===>    Group 1 = “abc”
         • 2nd Match at “def” ===>      Group 1 = “def”
           • 2nd match will override the value of 1st match for GROUP 1

Capturing Groups Names

1. Some RegEx engins allow to name the Capturing Group. [Still working on this part]