# $format = "/[12345]/";           #  I expect strings like this
# $format = "/[[12345]//";         #  match "[/"     :-]
# $format = "/aa/[[12345]//";      #  match "aa/[/"

my $success = 1;
my $test = "x";
if ( $format =~ m!^/(.*)/$! ) {    # test for "/reg.expr./" format
    eval { $test =~ m/$1/ };       # test of "user" reg. exp.
    if ( $@ ) {
        $success = 0;              # $format is incorrect reg. expr.
    }
}