I would like to create a String.replaceAll() method in JavaScript and I'm thinking that using a regex would be most terse way to do it. However, I can't figure out how to pass a variable in to a.

Certainly! To create a custom replaceAll() method in JavaScript using regular expressions, you can define a function that takes the string, the pattern to replace, and the replacement value as arguments. Then, you can use the replace() method along with a regular expression to perform the replacement globally. Here's an example:

function customReplaceAll(inputString, find, replaceWith) {
    const regex = new RegExp(find, 'g');
    return inputString.replace(regex, replaceWith);
}

const originalString = "Hello, world! How are you, world?";
const findPattern = "world";
const replacement = "universe";

const modifiedString = customReplaceAll(originalString, findPattern, replacement);
console.log(modifiedString);

In this example, the customReplaceAll() function takes the input string, the pattern to find, and the replacement value as arguments. It creates a regular expression with the 'g' flag to match globally (replace all occurrences), and then uses the replace() method to perform the replacement.

When you run the code, the output will be:

Hello, universe! How are you, universe?

Make sure to be cautious when using dynamic values in regular expressions, especially if those values are coming from user input or other untrusted sources. You might need to escape any special characters in the pattern before constructing the regular expression.