Question

Please explain the lexical analysis of the following grammar: b* a b* a (a+b)*

For example, this can generate the word: bbabbbabbaa since the first b* can be used to generate two b's followed, then an 'a', followed by three b's in the second b* followed by an 'a', then (a+b)* can be used to generated a 'b', 'b', 'a', and an 'a'.

Please explain in simple terms as this topic is confusing. What more can be said about the grammar?

Answer 1

Actually  b* a b* a (a+b)* is not a grammar but a regular expression

Let me break it down

b* can generate zero or more b's

a can generate only a single a

b* can generate zero or more b's

a can generate only a single a

(a+b)* can generate any combination of a's and b's including (empty string)

The shortest string that it can generate is

a a

In short, the expression generates strings that contain atleast two a's.

You can never generate strings that contain less than two a's.