javascript - Regex expression with 1 references not matching desired string - Stack Overflow

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I am working on this regex expression and i have confusion how it is working.

/^(\d+)\s\1\s\1$/

i know that

^ is start of line 
() shows capture group
\d+ shows digits 
\s is whitespace 
\1 shows to match first group
$ the end 

confusing part is $ here and why it is matching to "40 40 40 " but not "40 40 40 40" and role of $

I am working on this regex expression and i have confusion how it is working.

/^(\d+)\s\1\s\1$/

i know that

^ is start of line 
() shows capture group
\d+ shows digits 
\s is whitespace 
\1 shows to match first group
$ the end 

confusing part is $ here and why it is matching to "40 40 40 " but not "40 40 40 40" and role of $

• javascript
• regex

Share Improve this question Follow edited Jan 23, 2020 at 4:19 Maria asked Jan 23, 2020 at 4:07 MariaMaria 5111 silver badge88 bronze badges 5

• 1 Hint: How many \1 instances are there? Could you make it match by adding more? – tadman Commented Jan 23, 2020 at 4:09
• two instance because i needed to match three groups not 4 . i am having issues how it works :) – Maria Commented Jan 23, 2020 at 4:13
• @Amadan are you saying $ is "nothing else " here – Maria Commented Jan 23, 2020 at 4:14
• Pretty much, yes. As you said, it is "end of string" (or "end of line", depending on the m flag), and ^ is "beginning of string" (or line). The pattern says there shall be three identical things sandwiched from start to end of string, with some spaces between them. If you omitted $, then "40 40 40foo" would find a match. If you omitted ^, then foo40 40 40" would. If you omit both, "foo40 40 40foo" would be okay. If you want three groups, and "40 40 40 40" is being rejected, then it's all as it should be, no? – Amadan Commented Jan 23, 2020 at 4:18
• pletely got it now usage of ^ and $ was confusing for me now i got it thank You .it is all .it is a challenge in FCC . you are awesome – Maria Commented Jan 23, 2020 at 4:23

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3 Answers 3

Sorted by: Reset to default 6

\1 refers to the first capturing group in the regular expression. \2 will refer to the second capturing group and \n will refer to an nth capturing group.

In the regular expression /^(\d+)\s\1\s\1$/. \1 will match the only group of digits which is matched by the first capturing group (\d+).

$ in /^(\d+)\s\1\s\1$/ Means it should end by one more digits.

^ in /^(\d+)\s\1\s\1$/ Means it should start by one more digits.

$ matches "end of string". The way you have the regex pattern defined, it shouldn't match either of your strings because 40 40 40 has an extra space at the end before the end of string and 40 40 40 40 has extra space and digits after the third 40 and before the end of string.

If you remove the $, then it will match both of your strings or any string that begins with 40 40 40, so for example it will also match 40 40 40 test.

I wrote the answer for this challenge as /^(\d+)(\s)\1\2\1$/g and it worked. If I'm correct, the first \1 will produce (number, space, number). The \2 will add the space making it (number, space, number, space). The final \1 adds the last number, making it (number\s number\s number). The ^ and $ start and end the regex.

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