Java separate string from one single string

I have String set which is "5X3I3M3X"

I want to separate 5X = 1X1X1X1X1X , depends on number in front of character "X"

So finally it could be 1X1X1X1X1X3I3M1X1X1X

I did it like below,

    String str = "5X3I3M3X";
    StringBuffer sBuffer = new StringBuffer("");

    for(int i = 1 ; i < str.length() ; i=i+2) {
        if( str.charAt(i) == 'X'){

            if(str.charAt(i-1) == 1){
                sBuffer.append(str.charAt(i-1)+""+str.charAt(i));
            }else{
                StringBuffer sBufferTemp = new StringBuffer("");

                for(int j=0 ; j < Character.getNumericValue(str.charAt(i-1)) ; j++){
                    sBufferTemp.append("1X");
                }

                sBuffer.append(sBufferTemp.toString());
            }
        } else {
            sBuffer.append(str.charAt(i-1)+""+str.charAt(i));
        }
    }
    System.out.println(sBuffer.toString());

And I want to know is there any simple way to separate that logic? note. If I want to add to separate 'I', then it should be dynamically change also like "1X1X1X1X1X1I1I1I3M1X1X1X"

I can change it my code extpend if (str.charAt(I) == 'I') ....

I hope there is another simple way to implement this.