Replacing lowercase CSS hex code with uppercase in vim

Changes #5e342f -> #5E342F or #efe -> #EFE

/#[0-9a-f]\{6}\|#[0-9a-f]\{3}/\U&/g

Breaking it down

Everything starts with #

[0-9a-f] is the range of characters

\{6} means that there needs to be exactly 6 occurrences of the pattern
NOTE: We're being more selective first since if the string isn't 6 characters long, it may only be 3.

\| is the pivot on the search it's VIM regex for OR.

Then #, [0-9a-f] as before.

\{3} catches those weird cases where the color code is only 3 characters long and repeats (e.g. #3f3, or #fff).

\U Changes all the alphabetic characters to uppercase.

& Applies the \U to the whole pattern.