After performing a regular expression such as:

if ('foo bar'.match(/(f\\S+)/)) {
    // do something
}

We can utilize RegExp.$1 to output whatever is caught in the regular expression. For example:

if ('foo bar'.match(/(f\\S+)/)) {
    alert(RegExp.$1); // alert "foo"
}

But if there is a certain jQuery selector parsing before the RegExp.$1, the RegExp.$1 might be changed unexpectedly. For example:

if ('foo bar'.match(/(f\\S+)/)) {
    $('div').addClass(RegExp.$1); // the added class become "div" instead of "foo"
}