[hardwired]

Code:

Exercise8_18.java:11: cannot find symbol
symbol  : variable str

Java cannot find a declaration for the variable "str" - it has not been declared.
You did however declare a string "s" and you saved the user input in it.

Code:

public class Exercise8_18 {
    /**Main method*/
    public static void main(String[] args) {
        // Receive text entered from the dialog box
        // Save it in local variable "s".
        String s = JOptionPane.showInputDialog("Enter a sentence " +
                                               "using punctuation:");
        String delims = "[\\s\\p{Punct}]+";
        // Split the String "s":
        String[] tokens = s.split(delims);  
                
        System.out.println("The extracted words are: " +
                            java.util.Arrays.toString(tokens));