Automation exam in the Faculty of Informatics Engineering, Damascus University

Question

What is the regular expression that accept following string aaaabbbb ؟
a) a+ b+
b) a* b*
c) (a+b)* (a+b)*
d) (a+b)*
e) abab

Comments

all except regular expression in option e) accepts aaaabbbb.

Answer 1

a)a$^{+}$b$^{+}$ means atleast one time a’s will repeat and atleast one time b’s will repeat,but they have to be one after the other.So the given string is generated by this regex.

b)a$^{*}$b$^{*}$  means zero or more times a’s will repeat and zero or more times b’s will repeat,but they have to be one after the other.So the given string is generated by this regex.

c)$(a+b)*(a+b)*$ We can ignore one $(a+b)*$ and let the other generate the given string

d)This is the set of all possible strings over Σ={ a,b} so it can generate all possible strings

e)This is a string itself it cannot generate the given string

Correct answer will be  $a,b,c,d$ .