2.1.3 [ECMA-262-1999] Section 7.1, Unicode Format-Control Characters
The Unicode format-control characters (i.e., the characters in category "Cf" in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages). It is useful to allow these in source text to facilitate editing and display.
#The format control characters can occur anywhere in the source text of an ECMAScript program. These characters are removed from the source text before applying the lexical grammar. Since these characters are removed before processing string and regular expression literals, one must use a. Unicode escape sequence (see 7.6) to include a Unicode format-control character inside a string or regular expression literal.#
JScript 5.x does not remove category Cf characters from the source text before applying the lexical grammar.