Fixed a reverse logic check in a validation (#3086)

Author: JWT007

log4j-core-test/src/test/java/org/apache/logging/log4j/core/config/CompositeConfigurationTest.java

@@ -293,11 +293,10 @@ public void evaluate() throws Throwable {
     private void runTest(final LoggerContextRule rule, final Statement statement) {
         try {
             rule.apply(
-                            statement,
-                            Description.createTestDescription(
-                                    getClass(),
-                                    Thread.currentThread().getStackTrace()[1].getMethodName()))
-                    .evaluate();
+                statement,
+                Description.createTestDescription(getClass(),
+                                                  Thread.currentThread().getStackTrace()[1].getMethodName()))
+                .evaluate();
         } catch (final Throwable e) {
             Throwables.rethrow(e);
         }

log4j-core/src/main/java/org/apache/logging/log4j/core/filter/RegexFilter.java

@@ -66,7 +66,7 @@ private RegexFilter(final Builder builder) {
 
         super(builder);
 
-        if (Strings.isNotBlank(builder.regex)) {
+        if (Strings.isBlank(builder.regex)) {
             throw new IllegalArgumentException("The 'regex' attribute must not be null or empty.");
         }
 
@@ -317,8 +317,8 @@ public Builder setUseRawMsg(final boolean useRawMsg) {
         public @Nullable RegexFilter build() {
 
             // validate the "regex" attribute
-            if (this.regex == null) {
-                LOGGER.error("Unable to create RegexFilter: The 'regex' attribute must be provided.");
+            if (Strings.isEmpty(this.regex)) {
+                LOGGER.error("Unable to create RegexFilter: The 'regex' attribute be set to a non-empty String.");
                 return null;
             }