How can I define a regular expression of a string which can start with Capital alphabet or integer (A-Z) or (0-9) and can be of any number of characters
I have tried [(A-Z)|(0-9)]* but its not working
could anyone please suggest?
The standard answer would be ^[A-Z0-9] , but nowadays it is better to use: ^[[:upper:][:digit:]]
A regular expression to be used by what utility? All systems have basic regular expressions, extended regular expressions, and pathname matching patterns. Some systems have additional types of regular expressions. Without knowing what utility you're going to use and the type of regular expressions it expects, we can't answer your question with an answer that we can be confident will do what you want.
If what you want is a regular expression to be used by grep (or several other utilities) to select lines in a file whose 1st character is an uppercase letter or a decimal digit, Scrutinizer's answer is what you want.