Don't put the regular expression in quotes, that makes it an ordinary string.

var s = "abcdwq'xx'x";
console.log(s.replace(/(["'])/g, "\\$1"));

Also, you were escaping the ] that ends [.

If you just want to escape single quotes, you don't need the brackets or capture group. Just do:

var s = "abcdwq'xx'x";
console.log(s.replace(/'/g, "\\'"));

What I have is scarcely better, but it does handle escaped backslashes too:

>>> var v= 'a\\"b'
>>> v
"a\"b"
>>> v.replace(/(\\*)(")/g, function(x) { var l = x.length; return (l % 2) ? x : x.substring(0,l-1) + '\\"' } )
"a\\"b"
>>> var v= 'a\\\\"b'
>>> v
"a\\"b"
>>> v.replace(/(\\*)(")/g, function(x) { var l = x.length; return (l % 2) ? x : x.substring(0,l-1) + '\\"' } )
"a\\"b"

If there are an odd number of slashes before a quote (1, 3, 5), the quote is already escaped; an even number (including zero), in needs escaping.

Made all the harder to read by the necessary of escaping the slashes in input and by the inability of the colorizer to understand the regexp expression...

Of course, you probably shouldn't even be doing this. If you have a raw string and you need something you can pass to (e.g.) eval, consider $.toJSON.

var myVar = '\"Things You Should Know\"';
document.write(myVar.replace(/\"/g, '|'));

The \ escapes the next character so your string only reads "Things You Should Know"

You have a string that contains double quotes, inside an array:

[ 'By.XPath("//test")' ]

Since you're using single quotes around double quotes, there is no issue. But if you used double quotes (which are functionally equivalent in JavaScript to single quotes), like this: "By.XPath("//test")", you would get a syntax error, because JavaScript would think you had two strings, "By.XPath" and ")", with some garbage in-between (//test). Removing special significance of a character is called "escaping", and is done in JavaScript using the backslash (not slash) character (\). Thus, to put a double quote inside a double-quoted string, you need to write "By.XPath(\"//test\")", where \" tells JavaScript that this " is not end of the string, but just a simple quote character.

Now, if you used copy(testData), you would get the exact string inside your clipboard: By.XPath("//test"). However, you copied an array that contains this string; this stringifies the array. When an array is turned into a string, you get brackets around comma-separated list of elements, and if an element is a string, it is double-quoted. So while you got around the need of escaping by using single quotes, JavaScript refuses to choose and always uses double quotes, and then has to use the double quotes to make sure the result is not garbage ("By.XPath("//test")").

Try this:

text = text.toString().replace(/"/g, '\\"')

Or this:

text = text.toString().replace('"', '\\"')

replace works for the first quote, so you need a tiny regular expression:

str = str.replace(/'/g, "\\'");