Search pattern in VIM

The search pattern of VIM is little different with Perl. There is one thing I igonred at the beginning, and it made me confused for a long time. That is the group pattern can't be used inside the "[]"

In http://vimdoc.sourceforge.net/htmldoc/usr_27.html#27.6

 item matches   equivalent 
\d digit   [0-9]
\D non-digit  [^0-9]
\x hex digit  [0-9a-fA-F]
\X non-hex digit  [^0-9a-fA-F]
\s white space  [  ]     ( and )
\S non-white characters [^  ]     (not  and )
\l lowercase alpha  [a-z]
\L non-lowercase alpha [^a-z]
\u uppercase alpha  [A-Z]
\U non-uppercase alpha [^A-Z]

Using this predefined ranges works a lot faster than the character range it stands for.
But they can't be used inside the "[]" .

:-)