I'm studying Regular Expressions by reading Aho's book. I don't understand two of the statements in the book:
Question A:
1(0+1)*1 + 1 : denotes the set of all strings beginning and ending with a 1.
My question why is +1 added at end of the regular expression? Shouldn't 1(0+1)*1 be sufficient?
I'm also having trouble with the following:
Question B:
The set of strings containing only 0's and 1's that have atmost one 1 as below
0*+0*10*
Can you explain how the solution 0*+0*10* is arrived at, step by step?
For Question a: 1(0+1)*1 denotes set of all strings beginning and ending with one but does not contain string 1 which has length one and starts and ends with one.
For Question b:
Set of strings containing atmost one 1 = A + B where
A is set of all strings containing zero 1s and
B is the set of all strings containing exactly one 1
So A is 0* and B is 0*10*
Hence we get the answer as 0* + 0*10*