I think what you are trying to know is the generic regular expression which starts with any alphabets and has only alpha-numeric characters in it.
Look, the generic format (in java, for string pattern matching) for your query is this:
String pattern = "[a-zA-Z][a-zA-Z0-9]*";
String pattern = "[a-zA-Z](\\w)*";
- represents kleene star operator which denotes the occurrences with one or more.
\w consists of the word characters.
Then further you can use pattern-matcher functions to do your work.