Fixed more sonarqube findings

src/main/java/com/mattrixwv/cipherstream/polysubstitution/PolybiusSquare.java

@@ -135,7 +135,7 @@ public class PolybiusSquare{
 		return cleanString;
 	}
 	protected String getPreparedInputStringDecoding(){
-		return inputString.replaceAll("[^0-9]", "");
+		return inputString.replaceAll("\\D", "");
 	}
 	//Strips invalid characters from the keyword and creates the grid
 	protected void setKeyword(String keyword){