Checks if a regular expression is an exact match for a string.
Parameters:
aRegexp -
A regular expression object or a string representing a regular
expression.
aString -
The string object against which the regular expression will be matched.
Returns:
True if aRegexp matches aString.
False otherwise.
The aRegexp can be either a string, whose contents will
be taken as a regular expression, or a regular expression object
returned by a call to the regexp-pattern function.
