A regular expression is used to match and tag words. It consists of a series of characters for matching strings.
Suppose, if A and B are regular expressions, then the following are true for them:
If {ɛ} is a regular language, then ɛ is a regular expression for it.
If A and B are regular expressions, then A + B is also a regular expression within the language {A, B}.
If A and B are regular expressions, then the concatenation of A and B (A.B) is a regular expression.
If A is a regular expression, then A* (A occurring multiple times) is also a regular expression.