[#...]                        Match Character Class


    A match in one character  can influence the match of  another character
    in another position  in the expression.   In the  first match character
    class, when the character in the class is matched, the position of  the
    character in the class is also noted.  When the second match  character
    class  is  reached,  only  that  character  in the same position may be
    matched.  In this manner, matched brackets or parenthesis may be found.
    The expression:

                      /[#\(\[\<\{]expression[#\)\]\>\}]/

    the  second   match  class   will  only   match  the   closing  bracket
    corresponding  to  the  matched  opening  bracket.    Matched character
    classes may not be nested.   There is no stacking  mechanism associated
    with match classes.