this just happens to be covered in the mod_rewrite URL Rewriting Guide (slightly modified for the situation in question):

RewriteEngine on
RewriteCond   %{HTTP_HOST}                 ^[^.]+\.perlmonks.org$
RewriteRule   ^(.+)                        %{HTTP_HOST}$1          [C]
RewriteRule   ^([^.]+)\.perlmonks.org(.*)  /home/monks/$1$2
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