$符号的特殊性

As ECMA’s define, there are some pattern in newString parameter, so, be careful about it.
(From ECMA-262 3rd Edition, but as test IE8 don’t support it.)
Specifying a string as a parameter

The replacement string can include the following special replacement patterns:

Pattern	Inserts
$$	Inserts a "$".
$&	Inserts the matched substring.
$`	Inserts the portion of the string that precedes the matched substring.
$'	Inserts the portion of the string that follows the matched substring.
$n or $nn	Where n or nn are decimal digits, inserts the n th parenthesized submatch string, provided the first argument was a RegExp object.