Regular expressions are mathematical symbolism
which describe the set of strings of specific language. It provides convenient
and useful notation for representing tokens. Here are some rules that describe
definition of the regular expressions over the input set denoted by ∑
1) € is a regular expression that denotes the set
containing empty string.
2) If R1 and R2 are regular expressions then R=R1+R2 (same can also be
represented as R=R1/R2)is also regular expression which represents union
operation.
3) If R1 and R2 are regular expressions then R=R1.R2 is also a regular
expression which represents concatenation operation.
4) If R1 is a regular expression then R=R1* is also a regular expression
which represents kleen closure.
A language
denoted by a regular expression is said to be a regular set or regular
language.
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