如何在JavaScript中转义和取消引用？

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?

Answer 1

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 (eg, 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.