Copying only files without a special string in filename I have to copy only files from a folder that don't have following string in its filename: NNNNxNNNN (a 3 or 4 digit number followed by the character x followed by a 3 or 4 digit number again -> it's the resolution of an image in the filename)

I'm trying for about an hour with the "find" command but it never does what it should do.
As find's -name does not support regular expressions you will have to explicitly define the four cases

find /source/ \
    -not -name '*[0-9][0-9][0-9]x[0-9][0-9][0-9]*'           -and \
    -not -name '*[0-9][0-9][0-9][0-9]x[0-9][0-9][0-9]*'      -and \
    -not -name '*[0-9][0-9][0-9]x[0-9][0-9][0-9][0-9]*'      -and \
    -not -name '*[0-9][0-9][0-9][0-9]x[0-9][0-9][0-9][0-9]*' \
    -exec cp {} /destination/ \;