SONARJAVA-5098 FP in S3457 when using strings involving \\n

Co-authored-by: leonardo-pilastri-sonarsource <[email protected]>

java-checks-test-sources/default/src/main/java/checks/PrintfMisuseCheckSample.java

@@ -454,3 +454,13 @@ void foo(org.slf4j.Logger log, org.slf4j.Marker marker) {
     log.debug("message");
   }
 }
+
+class SonarJava5098 {
+  void foo() {
+    String.format("\\newpage %n"); // Compliant
+    String.format("\\\newpage %n"); // Noncompliant {{%n should be used in place of \n to produce the platform-specific line separator.}}
+    String.format("\newpage %n"); // Noncompliant
+    String.format("aaa\\naaa\\\\naaa\\\\\na%naaa\\\\\\n"); // Noncompliant
+    String.format("aaa\\naaa\\\\naaa\\\\na%naaa\\\\\\n"); // Compliant
+  }
+}

java-checks/src/main/java/org/sonar/java/checks/PrintfMisuseCheck.java

@@ -391,9 +391,22 @@ private void checkBoolean(MethodInvocationTree mit, String param, Type argType)
   }
 
   private void checkLineFeed(String formatString, MethodInvocationTree mit) {
-    if (formatString.contains("\\n")) {
-      reportIssue(mit, "%n should be used in place of \\n to produce the platform-specific line separator.");
+    var index = formatString.indexOf("\\n");
+    while (index != -1) {
+      if (isOddNumberOfEscapeChars(formatString, index)) {
+        reportIssue(mit, "%n should be used in place of \\n to produce the platform-specific line separator.");
+        return;
+      }
+      index = formatString.indexOf("\\n", index+2);
+    }
+  }
+
+  private static boolean isOddNumberOfEscapeChars(String formatString, int lastIndex) {
+    var index = lastIndex-1;
+    while (index >= 0 && formatString.charAt(index) == '\\') {
+      index--;
     }
+    return (lastIndex - index) % 2 != 0;
   }
 
   private static boolean usesMessageFormat(String formatString, List<String> params) {