Good News About Retrosheet...

mod_rewrite is a very powerful plug in for the Apache webserver. It is consequently also very confusing and complicated.

The first line reads:
RewriteMap uppercase int:toupper

This defines a mapping function called 'uppercase' which uses Apache's internal uppercasing function. I didn't know this existed until I looked it up today. You could call the function whatever you wanted - "foobar" or "doodoohead" or whatever, but obviously it's easier if you name it something descriptive.

The second line reads:
RewriteRule ^boxesetc/(P{1}(a-z{1}).*) boxesetc/${uppercase:$2}/$1 [R=301, L]

This is where the magic happens. It defines a rule to be invoked if part of the URL matches the pattern ^.....*) which will then be substituted with boxesetc/$....$1

Pattern:
^ denotes start of a string.
boxesetc/ is included at the start of the string just for sanity's sake.
Then we group the remainder using parentheses. This will be recalled as a backreference in the substitution as $1.
P{1} means match a single capital P. All personal pages in old (and new) retrosheet URLs start with P, as far as I can tell.
Then we group the next character using parentheses to create backreference $2. We match a-z{1} meaning a single lowercase letter.
Then .* should match all remaining characters in the string.

Substitution:
boxesetc/ is again included since we included it before.
${uppercase:$2} returns the string created by feeding backreference $2 into the function 'uppercase'. In the case of the Sandy Koufax page (Pkoufs101.htm) this would feed the lowercase letter k into the 'uppercase' function and return capital K.
/ is the directory delimiter.
$1 returns backreference $1, or Pkoufs101.htm

Pretty complicated, huh?