Preg Match Empty String

Question

How to preg_match for an null (empty) string??

I need something like:

/([0-9]|[NULL STRING])/

Answer 1

I had to do this, in java, for the last 4 for a CC number. The field is optional so it could either be empty, or 4 digits. My solution is:

^\d{4}$|^$

The is passing this a null in the typical java fashion results in a null pointer exception:

String myString = null;
last4Pattern.matcher(myString).matches(); //Null in this case.

However, I feel that it is more of a Java implementation problem.