Re: Oh, forgot other question regarding regular expressions
niclane wrote:
Perhaps this is connected with my misunderstanding that has lead to my
earlier post but with regular expressions in the Sipser book definition
1.52 states:
that R is a regular expression if R is:
...
2. "empty"
...
Among other possibilities, yes.
then on the next page is states:
"R[upperscript +] = RR*."
then later
"R[upperscript +] union "empty" = R*
My question is why is R[upperscript +] not already have "empty" in it?
Even though it must have 1 or more concatenations of R doesn't R have
empty in it? And so can't R concat R be potentially "empty" concat
"empty"?
R need not denote a language with the empty string. It might, but
it certainly won't always. For example, if R = {a},
R^+ = {a, aa, aaa, ...}
and R^+ obviously won't contain the empty string.
Regards,
Rick
.
