Well, you can use this:

var reg = new RegExp(oldWord, "g");
myString.replace(reg, "");

or simply:

myString.replace(new RegExp(oldWord, "g"), "");

A simple solution is not to use RegEx at all. Use Template literals

var module = 'm1',
    taskId = 't1',
    hash   = 'h1';

var url = `/task/${module}?taskId=${taskId}#${hash}`;

var module = 'm1',
    taskId = 't1',
    hash = 'h1';

var url = `/task/${module}?taskId=${taskId}#${hash}`;

document.body.innerHTML = url;

Using RegEx:

function replaceUrl(url, data) {
    // Create regex using the keys of the replacement object.
    var regex = new RegExp(':(' + Object.keys(data).join('|') + ')', 'g');

    // Replace the string by the value in object
    return url.replace(regex, (m, $1) => data[$1] || m);
}

function replaceUrl(url, data) {
    var regex = new RegExp(':(' + Object.keys(data).join('|') + ')', 'g');

    return url.replace(regex, (m, $1) => data[$1] || m);
}


var updatedUrl = replaceUrl('/task/:module?taskId=:taskId#:hash', {
    module: 'm1',
    taskId: 't1',
    hash: 'h1'
});

console.log(updatedUrl);
document.body.innerHTML = updatedUrl;

Your current regex extracts all the text contained within {{}} with its capture group. But you only want the index of the replacement, which is contained within the [], and not the entire string itself. So you have two options:

• Modify regex to capture only the index, so that would look like /{{field\[(.+?)\]}}/, where the capture group now only takes the number within the brackets.
• Leave the original regex alone, but change the replace function to extract the number from the returned match. In this case you'll have a second regex (or some other method) to extract the number from the matched string (in this case, get "10" out of "field[10]").

Here's an example demonstrating both:

var string = 'This is some content: {{field[10]}}';
var submission = {inputs: []};
submission.inputs[10] = 'replace value';

// I want the new string to be this
// var newString = 'This is some content: replace value';

var newString = string.replace(/{{field\[(.+?)\]}}/g, (match, cap1) => submission.inputs[cap1]);
console.log(newString)

// OR:

var otherNewString = string.replace(/\{{(.+?)}}/g, (match, cap1) => submission.inputs[cap1.match(/\[(.+?)\]/)[1]]);
console.log(otherNewString)

string.replace(":docsID",var)

To easy right? Next time try first!

You can achieve this by using Template Literals.

In your case:

const a = 1;
const b = 2;
const result = `a is ${a}, b is ${b}`; // a is 1, b is 2

You just need to write your string like this: `My string`

And to concatenate a variable value, you write the variable like this: ${myVariable}

So, the final result would look like this:

const myVariable = 'awesome';
const finalResult = `My string is ${myVariable}` // My string is awesome

The RegExp constructor takes a string and creates a regular expression out of it.

function name(str,replaceWhat,replaceTo){
    var re = new RegExp(replaceWhat, 'g');
    return str.replace(re,replaceTo);
}

If replaceWhat might contain characters that are special in regular expressions, you can do:

function name(str,replaceWhat,replaceTo){
    replaceWhat = replaceWhat.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
    var re = new RegExp(replaceWhat, 'g');
    return str.replace(re,replaceTo);
}

See Is there a RegExp.escape function in Javascript?

If I understand correctly for this to work as dynamically as you state you will have to do the following

// example variable, you need to use var so its 
// available on the window otherwise this will not work
var categoryName = "movies"; 
...
let searchUrl = "category/[categoryName]/all";
let regex = /\[(.+?)\]/ug;
let variableName = searchUrl.match(regex)[0];
searchUrl = searchUrl.replace(regex, window['variableName']);

Your dynamic variable will have to be stored globally for this work!