Node.js: Replace a second part of a pattern in a text file line-by-line

I've a less file with content like this:

@background: #345602;
@imgBack: #000;

I can read the whole text file into a variable, and latter save the content of the variable modifying the file:

var lessStr = grunt.file.read('./myLessFile.less');

Now I want to change the variable, say, @imgBack to #ff0000. So that, the modified file would look like:

@background: #345602;
@imgBack: #ff0000;

I there any way to do this by regular expression match and replace? Please help.

EDIT

I've code like:

var str = '@black:                 #000;\n@grayDarker:            #222;\n@grayDark:              #333;\n@gray:                  #555;\n@grayLight:             #999;';
var varName = '@black';
var replace = '#ab4564';
var regex = '(' + varName + ':\\s*)(?:#[a-z0-9]+)(.*)$';
var re = new RegExp(regex, 'm');
var replaceStr = '$1' + replace;
str.replace(re, replaceStr);
console.log(str);

But it's not working. Have I mistaken something.