Backreferences in pattern: \N and \k<name>

We can use the contents of capturing groups pattern:(…) not only in the result or in the replacement string, but also in the pattern itself.

Backreference by number: \N

A group can be referenced in the pattern using pattern:\N, where N is the group number. To make clear why that’s helpful, let’s consider a task. We need to find quoted strings: either single-quoted subject:‘…’ or a double-quoted subject:“…” – both variants should match. How to find them? We can put both kinds of quotes in the square brackets: pattern:‘“[’”], but it would find strings with mixed quotes, like match:“…’ and match:‘…”. That would lead to incorrect matches when one quote appears inside other ones, like in the string subject:“She’s the one!”: As we can see, the pattern found an opening quote match:“, then the text is consumed till the other quote match:’, that closes the match. To make sure that the pattern looks for the closing quote exactly the same as the opening one, we can wrap it into a capturing group and backreference it: pattern:([‘”])(.*?)\1. Here’s the correct code: Now it works! The regular expression engine finds the first quote pattern:([’“]) and memorizes its content. That’s the first capturing group. Further in the pattern pattern:\1 means “find the same text as in the first group”, exactly the same quote in our case. Similar to that, pattern:\2 would mean the contents of the second group, pattern:\3 - the 3rd group, and so on.

Backreference by name: \k

If a regexp has many parentheses, it’s convenient to give them names. To reference a named group we can use pattern:\k. In the example below the group with quotes is named pattern:?, so the backreference is pattern:\k:

let str = `He said: "She's the one!".`;

let regexp = /['"](.*?)['"]/g;

// The result is not what we'd like to have
alert( str.match(regexp) ); // "She'