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For example if = = {a} and we have regular expression R = a®, then R is a set denoted by R = {e, a, aa, ana, aaaa, ...} That is R includes any number of a's as well as empty string which indicates zero number of a's appearing, denoted by € character. Similarly there is a positive closure of L which can be shown as L*. The L* denotes set of all the strings except the € or null string. The null string can be denoted eor* ” If E = {a} and if we have regular expression R=a* then R is a set denoted by R = {a, aa, aaa, aaaa, ....} We can construct L* as L* = e-Lt Let us try to use regular expressions with the help of some examples. ‘mp Example 3.1: Write the regular expression for the language accepting all combinations of a's over the set & ={a}. Solution : All combinations of a's means a may be single, double, tripple and so on. There may be the case that a is appearing for zero times, which means a null string. That is we expect the set of {e, a, aa, aaa, ...). So we can give regular expression for this as R= a That is kleen closure of a. 
