Specific message pattern causes ArrayIndexOutOfBoundsException throwed by ParameterizedMessage.init()

[hucsmn]

Description

Message pattern "\\\\{}" causes ArrayIndexOutOfBoundsException during ParameterizedMessage.init()

Configuration

Default spring boot configuration.

Version: [Log4j version]

Known reproducable versions: 2.12.0 - 2.20.0

Operating system: [OS and version]

Windows 10

JDK: [JDK distribution and version]

Oracle JDK 1.8u362

Logs

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2
	at org.apache.logging.log4j.message.ParameterFormatter.countArgumentPlaceholders2(ParameterFormatter.java:121)
	at org.apache.logging.log4j.message.ParameterizedMessage.init(ParameterizedMessage.java:149)
	at org.apache.logging.log4j.message.ParameterizedMessage.<init>(ParameterizedMessage.java:123)
	at org.apache.logging.log4j.message.ParameterizedMessage.<init>(ParameterizedMessage.java:132)
	at org.apache.logging.log4j.message.ParameterizedMessageFactory.newMessage(ParameterizedMessageFactory.java:70)
	at org.apache.logging.log4j.spi.AbstractLogger.logMessage(AbstractLogger.java:2033)
	at org.apache.logging.log4j.spi.AbstractLogger.logIfEnabled(AbstractLogger.java:1899)
	at org.apache.logging.slf4j.Log4jLogger.error(Log4jLogger.java:304)
        ......

Reproduction

package org.apache.logging.log4j.message;

public class Test {
    private static final String testMsg1 = "\\\\{}";
    private static final String testMsg2 = "\\\\{} blabla";
    private static final String[] testArgs = {"a"};

    public static void main(String[] args) {
        System.out.println(formatUsingReusableParameterizedMessage(testMsg2));  // \a blabla
        System.out.println(formatUsingParameterizedMessage(testMsg2));          // \a blabla
        System.out.println(formatUsingReusableParameterizedMessage(testMsg1));  // \a
        System.out.println(formatUsingParameterizedMessage(testMsg1));          // throws ArrayIndexOutOfBoundsException
    }

    public static String formatUsingParameterizedMessage(final String msg) {
        return new ParameterizedMessage(msg, testArgs).getFormattedMessage();
    }

    public static String formatUsingReusableParameterizedMessage(final String msg) {
        return new ReusableParameterizedMessage().set(msg, testArgs).getFormattedMessage();
    }
}

[vy]

@hucsmn, thanks for the report! This is indeed a bug. I will work on it.