Greetings all,
Though a little more code to illustrate what you have tried up to now might be helpful my intuitive feeling is that $b is a row vector and could therefore be addressed as such. No need for splitting and regexp. Im not sure if you are using PDL::Matrix or PDL::MatrixOps but I know there is an issue with element addressing being either (row, column) or (column, row). I would suggest reading up on PDL::Indexing before rolling out a regexp solution for this, slice should suffice.

-InjunJoel