This is not a homework question, just a general question. I'm trying to study Regular Expressions for an exam. This is taken from a textbook(Introduction to the Theory of Computation by Sipser):
For convenience, we let R+ be shorthand for RR∗. In other words, whereas R∗ has all strings that are 0 or more concatenations of strings from R, the language R+ has all strings that are 1 or more concatenations of strings from R. So R+ ∪ ε = R∗. In addition, we let Rk be shorthand for the concatenation of k R's with each other.
I don't really understand the meaning of R+. Can anyone explain and give an example? Thank you