How can I replace a regex substring match in Javascript?

Question

var str   = 'asd-0.testing';
var regex = /asd-(\d)\.\w+/;

str.replace(regex, 1);

That replaces the entire string str with 1. I want it to replace the matched substring instead of the whole string. Is this possible in Javascript?

Solution 1

var str   = 'asd-0.testing';
var regex = /(asd-)\d(\.\w+)/;
str = str.replace(regex, "$11$2");
console.log(str);

Or if you're sure there won't be any other digits in the string:

var str   = 'asd-0.testing';
var regex = /\d/;
str = str.replace(regex, "1");
console.log(str);

Solution 2

I would get the part before and after what you want to replace and put them either side.

Like:

var str   = 'asd-0.testing';
var regex = /(asd-)\d(\.\w+)/;

var matches = str.match(regex);

var result = matches[1] + "1" + matches[2];

// With ES6:
var result = `${matches[1]}1${matches[2]}`;

Solution 3

I think the simplest way to achieve your goal is this:

var str   = 'asd-0.testing';
var regex = /(asd-)(\d)(\.\w+)/;
var anyNumber = 1;
var res = str.replace(regex, `$1${anyNumber}$3`);

Comments

[eduncan911]

+1 I personally like having the collection of matches to frack with.

[Travis J]

I agree, having the matched set available is more readable in my opinion.

[Ivan Rave]

or using function: 'asd-0.testing'.replace(/(asd-)\d(\.\w+)/, function(mystring, arg1, arg2){return arg1 + 'mynumber' + arg2})

[Martin Massera]

is there any answer where you DONT know the structure of the regex? here you are basically creating a new regex with two matches

[Bas Slagter]

It is good to know that you need braces () around the part you want as $1, $2 etc.