In terms of region file names

### overworld:
x < -20 && y < -4
x < -22
x < -9 && 1 < y < 8
-19 < x < -3 && y < -2
x > 5 && y < -7
x > 9 && y < 5
-9 < x < 7 && y > 3

### nether:
x < -3 || y < -7

######### Execution:

~/.minecraft/saves/world $ find DIM1 -regex '.*/r\..*\.mca' -exec bash -c "echo {} | ./region_check.sh" \; -print
   *** prints files to delete, replace -print in the command above with -rm to actually rm them **
~/.minecraft/saves/world $ cat region_check.sh 
#!/usr/bin/env bash

IFS=. read dummy x y dummy
#echo $x $y

if [ $(bc <<<"$x < -20 && $y < -4 || $x < -22 || $x < -9 && 1 < $y && $y < 8 || -19 < $x && $x < -3 && $y < -2 || $x > 5 && $y < -7 || $x > 9 && $y < 5 || -9 < $x && $x < 7 && $y > 3") == "1" ]; then 
  exit 0  
else
  exit 1
fi