Question

I am currently using the following regular expression:

^[a-zA-Z]{0,}(\\*?)?[a-zA-Z0-9]{0,}

to check a string to start with an alpha character and end with alphanumeric characters and have an asterisk(*) anywhere in the string but only a maximum of one time. The problem here is that if the given string still passes if it starts with a number but doesn’t have an *, which should fail. How can I rework the regex to fail this case?

ex.

TE - pass

*TE - pass

TE* - pass

T*E - pass

*9TE - pass

*TE* - fail (multiple asterisk)

9E - fail (starts with number)

EDIT:
Sorry to introduce a late edit but I also need to ensure that the string is 8 characters or less, can I include that in the regex as well? Or should I just check the string length after the regex validation?

Answer 1

This passes your example:

"^([a-zA-Z]+\\*?|\\*)[a-zA-Z0-9]*$"

It says:
  start with: [a-zA-Z]+\\*? (a letter and maybe a star)
              | (or)
              \\* a single star
  and end with [a-zA-Z0-9]* (an alphanumeric character)

Code to test it:

public static void main(final String[] args) {
    final Pattern p = Pattern.compile("^([a-zA-Z]+\\*?|\\*)\\w*$");

    System.out.println(p.matcher("TE").matches());
    System.out.println(p.matcher("*TE").matches());
    System.out.println(p.matcher("TE*").matches());
    System.out.println(p.matcher("T*E").matches());
    System.out.println(p.matcher("*9TE").matches());
    System.out.println(p.matcher("*TE*").matches());
    System.out.println(p.matcher("9E").matches());
}

Per Stargazer, if you allow alphanumeric before the star, then use this:

^([a-zA-Z][a-zA-Z0-9]*\\*?|\\*)\\w*$