When using parameter prompting, you won't need the backslash. 8.3.5 Region filters A region filter consists of a name followed by an equals sign followed by a region expression. The name must refer to a two-dimensional quantity. The region elements define shapes in a two-dimensional plane. Each region element name also has an abbreviated form.
Angles are in degrees.
Filter commands A filter specification may be either a range filter, a relational filter, or a region filter. A standard filter command consists of only a single filter component. The data subspace of the specified block is opened and used to filter the input block. The select command selects columns for an output table. For an image, it specifies the names and ordering of output axes, and is interpreted as a bin command.