replace all 3-character HEX color codes with 6-character codes in vim

gistfile1.txt

# this will replace all 3-character HEX color codes with 6-character codes in vim.

%s/#\([0-9A-Fa-f]\)\([0-9A-Fa-f]\)\([0-9A-Fa-f]\)\([0-9A-Fa-f]\)\@!/#\1\1\2\2\3\3/g
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   2---------------------------------------------
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1. substitution.
2. # followed by valid hex digit. \( \) is used to group the matches.
3. zero width preceding match to only match #111, not #111111. ':help @!' for details.
4. substitution. double each of the matched groups.
5. global match.