function reUtf16UnpairedSurrogate() {
	return /(?:[^\uD800-\uDBFF]|^)[\uDC00-\uDFFF]|[\uD800-\uDBFF](?![\uDC00-\uDFFF])/;
}
Returns a regular expression to match a UTF-16 unpaired surrogate.

@returns {RegExp} regular expression

@example
var RE_UTF16_UNPAIRED_SURROGATE = reUtf16UnpairedSurrogate();

var bool = RE_UTF16_UNPAIRED_SURROGATE.test( '\uD800' );
// returns true

bool = RE_UTF16_UNPAIRED_SURROGATE.test( '\uDC00' );
// returns true

bool = RE_UTF16_UNPAIRED_SURROGATE.test( 'abc' );
// returns false