PatternSyntaxException是未检查异常，它指示正则表达式模式中的语法错误。PatternSyntaxException类提供以下方法来帮助您确定出了什么问题：

    以下源代码RegexTestHarness2.java，更新了我们的测试工具，以检查格式错误的正则表达式：

    1. import java.io.Console;
    2. import java.util.regex.Pattern;
    3. import java.util.regex.Matcher;
    4. import java.util.regex.PatternSyntaxException;
    6. public class RegexTestHarness2 {
    8.     public static void main(String[] args){
    9.         Pattern pattern = null;
    10.         Matcher matcher = null;
    12.         Console console = System.console();
    13.         if (console == null) {
    14.             System.err.println("No console.");
    15.             System.exit(1);
    16.         }
    17.         while (true) {
    18.             try{
    19.                 pattern = 
    20.                 Pattern.compile(console.readLine("%nEnter your regex: "));
    22.                 matcher = 
    23.                 pattern.matcher(console.readLine("Enter input string to search: "));
    24.             }
    25.             catch(PatternSyntaxException pse){
    26.                 console.format("There is a problem" +
    27.                                " with the regular expression!%n");
    28.                 console.format("The pattern in question is: %s%n",
    29.                                pse.getPattern());
    30.                 console.format("The description is: %s%n",
    31.                                pse.getDescription());
    32.                 console.format("The message is: %s%n",
    33.                                pse.getMessage());
    34.                 console.format("The index is: %s%n",
    35.                                pse.getIndex());
    36.                 System.exit(0);
    37.             }
    38.             boolean found = false;
    39.             while (matcher.find()) {
    40.                 console.format("I found the text" +
    41.                     " \"%s\" starting at " +
    42.                     "index %d and ending at index %d.%n",
    43.                     matcher.group(),
    44.                     matcher.start(),
    45.                     matcher.end());
    46.                 found = true;
    47.             }
    48.             if(!found){
    49.                 console.format("No match found.%n");
    50.             }
    51.         }
    52.     }
    53. }

    要运行此测试，请输入?i)foo作为正则表达式。该错误是程序员在写标志表达式(?i)时，忘记了左括号的常见情况。这样做将产生以下结果：

    1. Enter your regex: ?i)
    2. There is a problem with the regular expression!
    3. The pattern in question is: ?i)
    4. The description is: Dangling meta character '?'
    5. The message is: Dangling meta character '?' near index 0
    6. ?i)
    7. ^
    8. The index is: 0

    从此输出中，我们可以看到语法错误是在索引0处悬空的元字符（问号）。导致开头括号缺失的原因。