How to negate pattern within sed?

Shell Programming and Scripting
1

Example:
I have data like,

H|1|2|#||4|4|5|6
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
T|g|g|G|G|g|g|
T|g|g|G|G|g|g|

I have to write command, it should delete all the lines except line starting with "D".

I have tried sed '/^\(D\)|/!d' filename . But its not working as expected.

Is there any other way of doing this?

2 
$ cat a.txt
H|1|2|#||4|4|5|6
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
T|g|g|G|G|g|g|
T|g|g|G|G|g|g|

$ sed '/^D/!d' a.txt
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
3 
$
$ cat a.txt
H|1|2|#||4|4|5|6
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
T|g|g|G|G|g|g|
T|g|g|G|G|g|g|
$
$ sed '/^[^D]/d' a.txt
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
D|f|g|h|j|j|k|k|
$
$