When trying to select an id/class that contains a backslash "\\", escaping doesn't seem to work. While "\\" and the characters that are escapable (".", "$", "[", "]", "/", etc.) aren't really valid in these attributes according to HTML 4.0, this is the only character that appears to be impossible to find using a selector. Any number of repeated "\\" characters appear to resolve to nothing when used as a selector. If this character is intended to be reserved in an id/class, mentioning so in the selector documentation and FAQ would resolve the issue.

Here is the markup in question:

<div id="test\\123"></div>

or

<div class="test\\123"></div>

Here are some selectors that fail to find either of these divs (but all of them would find <div id="test123"></div> or <div class="test123"></div>, if it were present in the markup):

$("#test123") // => 0 items (0 expected)
$("#test\\\\123") // js-escaped as "#test\\123", selector-escaped as "#test123" => 0 items (0 expected)
$("#test\\\\\\123") // js-escaped as "#test\\123", selector-escaped as "#test123" => 0 items (0 expected)
$("#test\\\\\\\\123") // ideal documentation case: js-escaped as "#test\\\\123", should be selector-escaped as "#test\\123" => 0 items (1 expected: fail)
$(".test\\\\\\\\123") // => 0 items (1 expected: fail) [others for class left out intentionally]

Any number of "\\" in the selector appear to simply resolve to doing a select without any "\\" characters (unless there is a different character to actually escape at the end, such as a "."):

$("#test\\\\\\\\\\\\\\\\\\\\\\\\123") // js-escaped as "#test\\\\\\\\\\\\123", should be selector-escaped as "#test\\\\\\123" => won't find <div id="test\\\\\\123"></div> but will find <div id="test123"></div> just like the other test cases