How do I make this preg_match case insensitive?

Just add the i modifier after your delimiter (# in your case):

preg_match("#(.{100}$keywords.{100})#i", strip_tags($description), $matches);

If your delimiter is /, add an i after it :

preg_match("/your_regexp_here/i", $s, $matches); // i means case insensitive

If the i modifier is set, letters in the pattern match both upper and lower case letters.