If you use

find pool -regextype posix-extended \
    -regex ".*/mypackage-([a-zA-Z0-9.]+-[0-9]{1,2})-x86_64\.pkg\.tar\.xz" \
    -exec printf '%f\n' {} + | \
egrep -o '([a-zA-Z0-9.]+-[0-9]{1,2})'

it should work for any path. The regex doesn't allow for any newlines, so there's no way to make it match for example a directory containing a similar name.

If you use

find pool -regextype posix-extended \
    -regex ".*/mypackage-([a-zA-Z0-9.]+-[0-9]{1,2})-x86_64\.pkg\.tar\.xz" \
    -printf '%f\n' |
  grep -Eo '[a-zA-Z0-9.]+-[0-9]{1,2}'

(assuming GNU grep as well), it should work for any path. The regex doesn't allow for any newlines, so there's no way to make it match for example a directory containing a similar name.

If you use find [...] -exec printf '%f\n' {} + | egrep -o '([a-zA-Z0-9.]+-[0-9]{1,2})' it should work for any path.

If you use

find pool -regextype posix-extended \
    -regex ".*/mypackage-([a-zA-Z0-9.]+-[0-9]{1,2})-x86_64\.pkg\.tar\.xz" \
    -exec printf '%f\n' {} + | \
egrep -o '([a-zA-Z0-9.]+-[0-9]{1,2})'

it should work for any path. The regex doesn't allow for any newlines, so there's no way to make it match for example a directory containing a similar name.