Show contents of a file without commented out lines and blank lines

A customer asked me how to display the contents of a text file, in 
this case a configuration file, without showing any blank line, nor 
any lines that started with a "#" character (commented out lines).

We can do this at the Linux command line, using sed, 
if you know how to craft the regex:

sed -e '/^ *#/d' -e '/^$/d' /dir/file.conf

For further reference:
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