How do I generate all possible numbers from this regular expression? [duplicate]

Question

I want to get a list of all possible values for a regular expression.

Input :

2W
9WW
7W0W3

where W can be any digit from 0 to 9. i.e. W = [0-9]

Output:

20,21,22,....29
900,901,...910,911,...999
70003,70013,70023,...71003,72003,...79093

What I did :

I'm using Java and decided to create an ArrayList of Integers.

I created a method ArrayList<Integer> getNumbers(String regex).

ArrayList<Integer> getNumbers(String regex){

ArrayList<Integer> fullList = new ArrayList<Integer>();

char[] cArray = regex.toCharArray(); //converted the string into a character array.

   for(int i=1;i<cArray.length;i++) {

       if(cArray[i] == 'W') {              

           for(int j=0;j<10;j++) {
               //I'm not sure what goes here
              fullList.add(the number with 'w' at this index replaced by 'j');
           }               
       }

   }
return fullList;
}

Is there any better way or library functions available to generate all such numbers?

How can I achieve this?

Answer 1

This is not quite a regex-based problem, but from an algorithmic perspective you can do the followings:

• Count the number of W's in your string.
• Based on the number of W's, create the product of range(0,9), for example if you have 2 W you need to create the products of two [0...9] lists that will be something like 0,0-0,1-0,2-...-9,9.
• Loop over the combinations and replace them with a simple string formatting. For instance when you are iterating over a triple combination suppose with 3 variables i,j,k you want want to replace them in a string like 7W0W3W, you can do "7%d0%dW%d"%(i,j,k).

And if you are looking for a general regex to wrap all the cases you can use a regex like (w) (w in a capture group) then you need to first access to position of the match groups and replace them with combination items (i,j,k,..).