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var format = "mm/dd/yyyy";
var match = new RegExp(format
    .replace(/(\w+)\W(\w+)\W(\w+)/, "^\\s*($1)\\W*($2)?\\W*($3)?([0-9]*).*")
    .replace(/m|d|y/g, "\\d"));
var replace = "$1/$2/$3$4"
    .replace(/\//g, format.match(/\W/));

function doFormat(target)
{
    target.value = target.value
        .replace(/(^|\W)(?=\d\W)/g, "$10")   // padding
        .replace(match, replace)             // fields
        .replace(/(\W)+/g, "$1");            // remove repeats
}

$(".birthdate-input").keyup(function(e) {
if(!e.ctrlKey && !e.metaKey && (e.keyCode == 32 || e.keyCode > 46))
    doFormat(e.target)
});
// don't let type letters into the birthdate input fields
$(".birthdate-input")[0].onkeypress = function(e) {
    e = e || event;
    if (e.ctrlKey || e.altKey || e.metaKey) return;
    var chr = getChar(e);

    if (chr == null) return;
    if (chr < '0' || chr > '9') {
        return false;
    }
}
function getChar(event) {
    if (event.which == null) {
        if (event.keyCode < 32) return null;
        return String.fromCharCode(event.keyCode) // IE
    }
    if (event.which != 0 && event.charCode != 0) {
        if (event.which < 32) return null;
        return String.fromCharCode(event.which) // other
    }
    return null; // special key
}

<input class="input birthdate-input" type="text" placeholder="mm/dd/yyyy" name="birthdate" maxlength="10"/>