how to get a string variable in regular expression code example

Example 1: javascript use variable regex

// CREATE A REGEXP object
var replace = "regex";
var re = new RegExp(replace,"g");
//then use
"mystring".replace(re, "newstring")

Example 2: add variable inside regex in javascript

var pattern="cd"
var repeats=3
new RegExp(`${pattern}{${repeats}}`, "g")