DFDL wish list for DFDL future versions (DFDL v2), suggests the need for the LSP (within a line only) character class entity, and more recently we've encountered the need for SP* and SP+ as well.
The LSP entity has 3 flavors: %LSP; %LSP*; and %LSP+; The "L" denotes within-a-line whitespaces, no line-ending characters, so really just space and tab.
The LSP matches exactly 1 such character, the LSP* matches zero or more, the LSP+ matches one or more.
On unparsing, the LSP and LSP+ output exactly 1 space. The LSP* outputs nothing.
The SP entity has 3 flavors also, but %SP; is already existing. %SP*; and %SP+; need to be created. These match zero or more, and one or more spaces (only) respectively. On unparsing SP* outputs nothing, SP+ outputs a single space.
This ticket is for implementing this new proposed/experimental feature for DFDL, which can be proposed for DFDL v2.0 inclusion
A dfdlx property must be used to enable this feature. Without setting this property, the feature should be disallowed. (The diagnostic message should clearly state how to enable it however. Rather than just saying it's not allowed.)
This feature requires a documentation page (wiki) also.