JavaScript - string regex backreferences

Question

You can backreference like this in JavaScript:

var str = "123 $test 123";
str = str.replace(/(\$)([a-z]+)/gi, "$2");

This would (quite silly) replace "$test" with "test". But imagine I'd like to pass the resulting string of $2 into a function, which returns another value. I tried doing this, but instead of getting the string "test", I get "$2". Is there a way to achieve this?

// Instead of getting "$2" passed into somefunc, I want "test"
// (i.e. the result of the regex)
str = str.replace(/(\$)([a-z]+)/gi, somefunc("$2"));

Answer 1

Using ESNext, quite a dummy links replacer but just to show-case how it works :

let text = 'Visit http://lovecats.com/new-posts/ and https://lovedogs.com/best-dogs NOW !';

text = text.replace(/(https?:\/\/[^ ]+)/g, (match, link) => {
  // remove ending slash if there is one
  link = link.replace(/\/?$/, '');
  
  return `<a href="${link}" target="_blank">${link.substr(link.lastIndexOf('/') +1)}</a>`;
});

document.body.innerHTML = text;

Answer 2

If you would have a variable amount of backreferences then the argument count (and places) are also variable. The MDN Web Docs describe the follwing syntax for sepcifing a function as replacement argument:

function replacer(match[, p1[, p2[, p...]]], offset, string)

For instance, take these regular expressions:

var searches = [
    'test([1-3]){1,3}',  // 1 backreference
    '([Ss]ome) ([A-z]+) chars',  // 2 backreferences
    '([Mm][a@]ny) ([Mm][0o]r[3e]) ([Ww][0o]rd[5s])'  // 3 backreferences
];
for (var i in searches) {
    "Some string chars and many m0re w0rds in this test123".replace(
        new RegExp(
            searches[i]
            function(...args) {
                var match = args[0];
                var backrefs = args.slice(1, args.length - 2);
                // will be: ['Some', 'string'], ['many', 'm0re', 'w0rds'], ['123']
                var offset = args[args.length - 2];
                var string = args[args.length - 1];
            }
        )
    );
}

You can't use 'arguments' variable here because it's of type Arguments and no of type Array so it doesn't have a slice() method.

Answer 3

Like this:

str.replace(regex, function(match, $1, $2, offset, original) { return someFunc($2); })

Answer 4

Note: Previous answer was missing some code. It's now fixed + example.

---

I needed something a bit more flexible for a regex replace to decode the unicode in my incoming JSON data:

var text = "some string with an encoded 's' in it";

text.replace(/&#(\d+);/g, function() {
  return String.fromCharCode(arguments[1]);
});

// "some string with an encoded 's' in it"

Answer 5

Pass a function as the second argument to replace:

str = str.replace(/(\$)([a-z]+)/gi, myReplace);

function myReplace(str, group1, group2) {
    return "+" + group2 + "+";
}

This capability has been around since Javascript 1.3, according to mozilla.org.