Question

This does not work because of the variable rep. What is the correct syntax please?

var bigtext = 'testing test test test';
var rep = 'test'; 
bigtext = bigtext.replace(/rep/g, "MOO!");

I know the problem is with the regex part in the replace…but what is the correct way to write it?

Answer 1

You need to build a regex using the RegExp constructor:

var bigtext = 'testing test test test';
var rep = 'test'; 
var regex = new RegExp(rep, 'g');
bigtext = bigtext.replace(regex, "MOO!");

Documentation for this constructor can be seen on the MDN page. Note that you probably should make sure that any special characters in regular expressions (e.g. [) are escaped.