use Regexp::Simple;

my $re = qr{
        start_of_line
        literal("1998/10/08")
        optional( whitespace )
        literal("[")
        remember( many( any( ":", digit ) ) )
        non_digit
};

if ('1998/10/08 [11:10]' =~ m/$re/) {
   print "Found a match at time $1!\n";
}