But remember that you only need to list those extensions that you actually use, unless you've got one of those case-insensitive servers. Do you really have image files named .jpg AND .JPG AND .jpe AND .jpeg and so on...?
CIDR ranges aren't very kind to Regular Expressions-- or vice versa-- so you may want to fudge it. For example if the offender lives at 188.8.131.52/20 just say ^12\.12\.\d\. without closing anchor, and to heck with anyone at 12.12.30.nn and .31.nn, or .48.nn and .49.nn. Unless they are particular friends of yours.