Why does the jQuery JSON parser need double escaping for backslashes?

Question

I have trouble wrapping my head around a peculiar feature of the JSON data format.

The situation is as follows: I have a string containing a Windows (sigh) directory path, backslashes escaped. For some reason, the jQuery JSON parser thinks that a single escape is not enough.

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script type="text/javascript">

var success = jQuery.parseJSON('{"a":"b:\\\\c"}');
var failure = jQuery.parseJSON('{"a":"b:\\c"}');

</script>

Can anyone explain what makes such double escaping necessary?

Solution

The first escape escapes it in the Javascript string literal.
The second escape escapes it in the JSON string literal.

The Javascript expression '{"a":"b:\\c"}' evaluates to the string '{"a":"b:\c"}'.
This string contains a single unescaped \, which must be escaped for JSON. In order to get a string containing \\, each \ must be escaped in the Javascript expression, resulting in "\\\\".