Regular Expression - Group (Capture|Substitution)

1 - About

group are regexp expression that normally capture the match during the parsing. You can then (extract|reference) the match content.

Groups are inside parentheses.

Look-around are also groups but implements an assertion and are not capturing the content

3 - Syntax

Every group must begin with an open bracket and end with a close bracket.


(myRegexp0 ( myRegexp1) ( myRegexp2) )

Construct Definition
(?<name>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

4 - Name

By define the group is indexed by id but you can give it a name with the following syntax


(?<name>X)

where X is the regular expression pattern that you want to capture

It's called a named-capturing group.

5 - 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)

6 - Non-Capturing

6.1 - 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.

6.2 - Look-around

7 - 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

8 - 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: Notepad++ - Replace with Regular Expression


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