Construct Definition (?X) | X, as a named-capturing group | ^ non-capturing ^^ | (?:X) | X, as a non-capturing group | | (?>X) | X, as an independent, non-capturing group | ^ Assertion (See Regexp - Look-around group (Assertion) - ( Lookahead | Lookbehind )) ^^ | (?=X) | X, positive lookahead (via zero-width) | | (?!X) | X, negative lookahead (via zero-width) | | (?<=X) | X, positive lookbehind (via zero-width) | | (?<!X) | X, negative lookbehind (via zero-width) | ^ Flag ^^ | (?idmsuxU-idmsuxU) | Nothing, but turns match flags i d m s u x U on - off | | (?idmsux-idmsux:X) | X, as a non-capturing group with the given flags i d m s u x on - off | ===== Name ===== By default, the group is indexed by index (0,1,2,…) but you can give it a name with the following syntax (?X)where X is the regular expression pattern that you want to captureIt's called a named-capturing group.
Index
Capturing groups are numbered by counting their opening parentheses from left to right. In the expression ((A)(B(C))), for example, there are the following groups:
  • 0 - Group zero always stands for the entire expression - ((A)(B(C)))
  • 1 - ((A)(B(C)))
  • 2 - (A)
  • 3 - (B(C))
  • 4 - (C)
Non-Capturing
Basic
A non capturing group will not be indexed.In the expression (?:A)(B)(C), for example, there are the following groups:
  • 0 - Group zero always stands for the entire expression - (?:A)(B)(C)
  • 1 - (B)
  • 2 - (C)
The group (?:A) was not captured.
Look-around
Regexp - Look-around group (Assertion) - ( Lookahead | Lookbehind )
Substitution
When you want to use the content of each captured group, you will generally use the following substitution construct:
  • n for the group index
  • groupName for the group name
When using group index, this construct must be used when:
  • the number of group is greater than 9
  • you want a number that follow the substitution
The dollar is also not always mandatory:
  • n for the group index
  • groupName for the group name
Their is also a shorthand notation for groups up to 9.
Symbol Definition
\0 backreference to the entire expression
\1 backreference to group 1
\2 backreference to group 2
\n backreference to group n
Example
The below regular expression has two groups 
([^ ]) (.*)
where:
  • the first group is [^ ] which will parse all non space characters.
  • the second group is .* which will take all characters.
if you parse the following text: 
Hello World
You will get:
  • in the first group, \1, the text Hello
  • and in the second group, \2, the text World
See more example here: A step by step on how to replace a text in Notepad++ with regular expression