A PatternSyntaxException is an unchecked exception that indicates a syntax error in a regular expression pattern. The PatternSyntaxException class provides the following methods to help you determine what went wrong:

• public String getDescription(): Retrieves the description of the error.
• public int getIndex(): Retrieves the error index.
• public String getPattern(): Retrieves the erroneous regular expression pattern.
• public String getMessage(): Returns a multi-line string containing the description of the syntax error and its index, the erroneous regular-expression pattern, and a visual indication of the error index within the pattern.

The following source code, RegexTestHarness2.java, updates our test harness to check for malformed regular expressions:

import java.io.Console;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
import java.util.regex.PatternSyntaxException;

public class RegexTestHarness2 {

    public static void main(String[] args){
        Pattern pattern = null;
        Matcher matcher = null;

        Console console = System.console();
        if (console == null) {
            System.err.println("No console.");
            System.exit(1);
        }
        while (true) {
            try{
                pattern = 
                Pattern.compile(console.readLine("%nEnter your regex: "));

                matcher = 
                pattern.matcher(console.readLine("Enter input string to search: "));
            }
            catch(PatternSyntaxException pse){
                console.format("There is a problem with the regular expression!%n");
                console.format("The pattern in question is: %s%n",pse.getPattern());
                console.format("The description is: %s%n",pse.getDescription());
                console.format("The message is: %s%n",pse.getMessage());
                console.format("The index is: %s%n",pse.getIndex());
                System.exit(0);
            }
            boolean found = false;
            while (matcher.find()) {
                console.format("I found the text \"%s\" starting at " +
                   "index %d and ending at index %d.%n",
                    matcher.group(), matcher.start(), matcher.end());
                found = true;
            }
            if(!found){
                console.format("No match found.%n");
            }
        }
    }
}

To run this test, enter ?i)foo as the regular expression. This mistake is a common scenario in which the programmer has forgotten the opening parenthesis in the embedded flag expression (?i). Doing so will produce the following results:

Enter your regex: ?i)
There is a problem with the regular expression!
The pattern in question is: ?i)
The description is: Dangling meta character '?'
The message is: Dangling meta character '?' near index 0
?i)
^
The index is: 0

From this output, we can see that the syntax error is a dangling metacharacter (the question mark) at index 0. A missing opening parenthesis is the culprit.