嘿，我想用下划线替换随机字母（仅第一个字母）。为此，我使用以下行：

String newSubstring=substring.replaceFirst(randomLetter,"_");

除非有括号“（”或“）”，否则效果很好。然后我得到以下异常：

06-14 15:29:48.090: E/AndroidRuntime(12466): FATAL EXCEPTION: main
06-14 15:29:48.090: E/AndroidRuntime(12466): Process: com.clozegenerator, PID: 12466
06-14 15:29:48.090: E/AndroidRuntime(12466): java.util.regex.PatternSyntaxException:     Incorrectly nested parentheses in regexp pattern near index 1:
06-14 15:29:48.090: E/AndroidRuntime(12466): )
06-14 15:29:48.090: E/AndroidRuntime(12466):  ^
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.util.regex.Pattern.compileImpl(Native Method)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.util.regex.Pattern.compile(Pattern.java:411)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.util.regex.Pattern.<init>(Pattern.java:394)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.util.regex.Pattern.compile(Pattern.java:381)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.lang.String.replaceFirst(String.java:1804)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at com.clozegenerator.MainActivity.generateCloze(MainActivity.java:138)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at com.clozegenerator.MainActivity.onClick(MainActivity.java:113)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at android.view.View.performClick(View.java:4480)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at android.view.View$PerformClick.run(View.java:18673)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at android.os.Handler.handleCallback(Handler.java:733)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at android.os.Handler.dispatchMessage(Handler.java:95)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at android.os.Looper.loop(Looper.java:157)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at android.app.ActivityThread.main(ActivityThread.java:5872)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.lang.reflect.Method.invokeNative(Native Method)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at java.lang.reflect.Method.invoke(Method.java:515)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1069)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:885)
06-14 15:29:48.090: E/AndroidRuntime(12466):    at dalvik.system.NativeStart.main(Native Method)

有什么想法可以解决吗？