How to escape and unescape quotes in JavaScript?

Question

Here's a short piece of code:

var utility = {
    escapeQuotes: function(string) {
        return string.replace(new RegExp('"', 'g'),'\\"');
    },
    unescapeQuotes: function(string) {
        return string.replace(new RegExp('\\"', 'g'),'"');
    }
};

var a = 'hi "';

var b = utility.escapeQuotes(a);
var c = utility.unescapeQuotes(b);

console.log(b + ' | ' + c);

I would expect this code to work, however as a result I receive:

hi \" | hi \"

If I change the first parameter of the new RegExp constructor in the unescapeQuotes method to 4 backslashes everything starts working as it should.

string.replace(new RegExp('\\\\"', 'g'),'"');

The result:

hi \" | hi " 

Why are four backslashes needed as the first parameter of the new RegExp constructor? Why doesn't it work with only 2 of them?

Solution

The problem is that you're using the RegExp constructor, which accepts a string, rather than using a regular expression literal. So in this line in your unescape:

return string.replace(new RegExp('\\"', 'g'),'"');

...the \\ is interpreted by the JavaScript parser as part handling the string, resulting in a single backslash being handed to the regular expression parser. So the expression the regular expression parser sees is \". The backslash is an escape character in regex, too, but \" doesn't mean anything special and just ends up being ". To have an actual backslash in a regex, you have to have two of them; to do that in a string literal, you have to have four (so they survive both layers of interpretation).

Unless you have a very good reason to use the RegExp constructor (e.g., you have to use some varying input), always use the literal form:

var utility = {
    escapeQuotes: function(string) {
        return string.replace(/"/g, '\\"');
    },
    unescapeQuotes: function(string) {
        return string.replace(/\\"/g, '"');
    }
};

It's a lot less confusing.