Regression expression to delete one or more spaces at end of string

R 3.3.1
OS X

Colleagues,

I have encountered an unexpected regex problem

I have read an Excel file into R using the readxl package.  Columns names
are:

COLNAMES        <- c("Study ID", "Test and Biological Matrix", "Subject
No. ", "Collection Date",
"Collection Time", "Scheduled Time Point", "Concentration", "Concentration
Units",
"LLOQ", "ULOQ", "Comment?)

As you can see, there is a trailing space in ?Subject No. ?.  I would like
to delete that space.  The following works:
        sub(? $?, ??, COLNAMES)
However, I would like a more general approach that removes any trailing
whitespace.

I tried variations such as:
        sub("[:blank:]$", "", COLNAMES)
(also, without the $ and ?space' instead of ?blank') without success ? to
my surprise, characters other than the trailing space were deleted but the
trailing space remained.

Guidance on the correct syntax would be appreciated.

Dennis

Dennis Fisher MD
P < (The "P Less Than" Company)
Phone / Fax: 1-866-PLessThan (1-866-753-7784)
www.PLessThan.com