CODEWITHSUNDEEP
javascriptregex
DNick
DNick

Reputation: 173

Expression Regular - Back References

I have this information:

My question. Why the 'back reference' doesn't work? I'd like to understand what the syntax does and where i did make the mistake. Because in my understanding the first group ([0123][0-9]) should consider both numbers '10' and '16', or not? why?

Regex Example Online

Upvotes: 1

Views: 75

Answers (2)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 627609

A backreference only matches the same text that was captured with the corresponding capturing group. It does not repeat the pattern, but the already captured value.

In your case, \1 tries to match 16 after the first -.

In JS regex, you can't use (?1) / \g<1> subroutine calls that can be used in PCRE/Onigmo, but you may define the subpattern as a variable and build the pattern dynamically:

var target = "16-10-2017";
var  p1 = "[0123][0-9]";
var rx = new RegExp("(" + p1 + ")-(" + p1 + ")-(\\d{4})", "g");
console.log(target.replace(rx, "$1/$2/$3"));

Upvotes: 3

Youcef LAIDANI
Youcef LAIDANI

Reputation: 60046

Maybe you have to change your regex to this (([0123][0-9])-){2}(\d{4}), which mean that it should match two part of numbers of two parts [0123] and [a-9] followed by - (([0123][0-9])-){2}

regex demo

Upvotes: 2

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