Regular Expression rrr represents a pattern of string, wherethe string set is called Regular Language The character is called Alphabet given an alphabet ∑\sum∑, we can construct regex rrrif r=a,a in ∑. L(r)={a}\text{if } r=a, a \text{ in } \sum. \text{ } L(r) = \{a\}if r=a,a in ∑. L(r)={a}if r=ɸ, L(r)={} // empty set\text{if } r=ɸ, \text{ } \text{ } L(r) = \{\} \text{ // empty set}if r=ɸ, L(r)={} // empty setif r=ε, L(r)={ε} // a set with an empty string\text{if } r=ε, \text{ } L(r)=\{ε\} \text{ // a set with an empty string}if r=ε, L(r)={ε} // a set with an empty string