A delimited item maple
can appear in a list in four ways
At the start of a list maple; oak
In the middle of a list oak; maple; birch
At the end of a list oak; maple
Alone with no delimiters maple
buildDelimRegex(str, delim)
A character string
A string whose presence indicates a selection
The delimiter separating the responses
This function builds a regex expression which captures those four cases It
is mostly broken out of createSubvarDeriv()
for testing purposes.