Updated working code

public String stringExpansionRegEx(String str) {

    StringBuilder result = new StringBuilder();

    boolean bool = Pattern.matches("([a-zA-Z]\\d)+", str);
    if (bool == true){
        for (int i=0;i<str.length();i+=2)  {
                 char ch = str.charAt(i);
                 int count = Integer.parseInt(String.valueOf(str.charAt(i+1)));
                 for (int j = 0; j < count; j++) {
                     result.append(ch);
                 }
        }
    }
    return result.toString();
}