Reguler Expression

Regular Expression have an important role in computer science applications. in application involving text, users may want to search for strings that satisfy certain pattern. regular expression provided a powerful method for describing such pattern such as AWK and GREP in UNIX.
Regular Expression is a set of symbols, Thus if alphabet= {a, b}, then aab, a, baba, bbbbb, and baaaaa would all be strings of symbols of alphabet.
Regular expressions can be used to define languages. A regular expression is like a "pattern"; strings that match the pattern are in the language, strings that do not match the pattern are not in the language. The construction of regular expressions is defined recursively, starting with primitive regular expressions, which can be composed using typical operators to form more complex regular expressions.
Regular languages are closed under many operations, including the three “regular operations”
1.Union
2.Concatenation:
3.Star (Kleene star):,
set intersection, set complement, string reversal, “homomorphism” (formal version of shifting alphabets). We
have seen (last class) why regular languages are closed under set complement.