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Backreferences

The contents of capturing groups can be referenced in the pattern using \N, where N is the group number or by \k<name>.

The common use cases for backreferencing are:

To find doubled words in a sentence as in "The sun rises in the the east".
To find words with doubled letters as in "successful", "bookkeeper".
To grab subtexts from texts separated by delimiters.

1. \N - By number
Word Wrap
Let's take an example to understand this feature. In the following panel, we take up a sentence and find and match all the repeated words in it. We capture each word using \b(\w+)\s+ and then refer the contents of the capturing group (\w+) using \1 later in the pattern to identify the duplicates.
Enter input string
I really appreciate it it if someone explains how the the engine process that pattern pattern step-by-step.
Enter regex pattern
i - Ignore Case
g - Global Match
m - Multiline
s - Dot All
u - Unicode
y - Sticky
With lastIndex^?
/\b(\w+)\s+\1\b/gCount:3
I really appreciate it it if someone explains how the the engine process that pattern pattern step-by-step.
match position $1
it it 20 it
the the 50 the
pattern pattern 78 pattern

match	position	$1
it it	20	it
the the	50	the
pattern pattern	78	pattern

2. \k<NAME> - By Name

Word Wrap

Using \k, we can assign convenient names to the captured groups instead of using the numbers.

Let's compose regexes to match (single or double) quoted strings. It should match both the variants.

Enter input string

Let's start with [''"](.*?)[''"]. But it could lead to incorrect matches in texts, where both the single and double quotes are present, like in the text "Fasten your seatbelts. It's going to be a bumpy night." Here the match starts at " and stops at '.

/['"](.*?)['"]/gCount:5

"Fasten your seatbelts. It's going to be a bumpy night." - All About Eve, 1950. "Frankly, my dear, I don't give a damn." - Gone With the Wind, 1939. 'Show me the money!' - Jerry Maguire, 1996. "There's no place like home." - The Wizard of Oz, 1939

match	position	$1
"Fasten your seatbelts. It'	0	Fasten your seatbelts. It
" - All About Eve, 1950. "	55	- All About Eve, 1950.
't give a damn."	104	t give a damn.
'Show me the money!'	149	Show me the money!
"There'	193	There

To make sure that the pattern matches the exact quote at both ends, we can capture and backreference it as given below. In this case, we used a name-based backreferencing using the name quote.

/(?<quote>['"])(.*?)\k<quote>/gCount:4

"Fasten your seatbelts. It's going to be a bumpy night." - All About Eve, 1950. "Frankly, my dear, I don't give a damn." - Gone With the Wind, 1939. 'Show me the money!' - Jerry Maguire, 1996. "There's no place like home." - The Wizard of Oz, 1939

match	position	$1	$2	quote
"Fasten your seatbelts. It's going to be a bumpy night."	0	"	Fasten your seatbelts. It's going to be a bumpy night.	"
"Frankly, my dear, I don't give a damn."	80	"	Frankly, my dear, I don't give a damn.	"
'Show me the money!'	149	'	Show me the money!	'
"There's no place like home."	193	"	There's no place like home.	"

We can achieve the same output with number-based backreferencing.

/(['"])(.*?)\1/gCount:4

"Fasten your seatbelts. It's going to be a bumpy night." - All About Eve, 1950. "Frankly, my dear, I don't give a damn." - Gone With the Wind, 1939. 'Show me the money!' - Jerry Maguire, 1996. "There's no place like home." - The Wizard of Oz, 1939

match	position	$1	$2
"Fasten your seatbelts. It's going to be a bumpy night."	0	"	Fasten your seatbelts. It's going to be a bumpy night.
"Frankly, my dear, I don't give a damn."	80	"	Frankly, my dear, I don't give a damn.
'Show me the money!'	149	'	Show me the money!
"There's no place like home."	193	"	There's no place like home.