sub lexer { 
    my($parser)=shift;
    my $s = $parser->YYData->{INPUT}; # reference to the string to lex
    m/\G\s+/gc;  skip any spaces
    return ('INT', $1) if $$s =~  m/\G\(d+)/gc;
    return ('ID',  $1) if $$s =~ m/\G([A-Z]\w*)/gc;
    ...  # and it goes on for many tentative matches
 }