NullPointerException at java.util.regex.Pattern, fixes #1224; from

com.puppycrawl.tools.checkstyle.checks.regexp.DetectorOptions.getPattern

src/main/java/com/puppycrawl/tools/checkstyle/checks/regexp/MultilineDetector.java

@@ -20,6 +20,8 @@
 package com.puppycrawl.tools.checkstyle.checks.regexp;
 
 import java.util.regex.Matcher;
+
+import com.google.common.base.Strings;
 import com.puppycrawl.tools.checkstyle.api.FileText;
 import com.puppycrawl.tools.checkstyle.api.LineColumn;
 
@@ -41,6 +43,12 @@ class MultilineDetector {
      */
     public static final String REGEXP_MINIMUM = "regexp.minimum";
 
+    /**
+     * A key is pointing to the warning message text in "messages.properties"
+     * file.
+     */
+    public static final String EMPTY = "regexp.empty";
+
     /** The detection options to use. */
     private final DetectorOptions options;
     /** Tracks the number of matches. */
@@ -65,9 +73,15 @@ class MultilineDetector {
     public void processLines(FileText text) {
         this.text = text;
         resetState();
-        matcher = options.getPattern().matcher(text.getFullText());
-        findMatch();
-        finish();
+
+        if (!Strings.isNullOrEmpty(options.getFormat())) {
+            matcher = options.getPattern().matcher(text.getFullText());
+            findMatch();
+            finish();
+        }
+        else {
+            options.getReporter().log(0, EMPTY);
+        }
     }
 
     /** recursive method that finds the matches. */

src/main/resources/com/puppycrawl/tools/checkstyle/checks/regexp/messages.properties

@@ -4,3 +4,5 @@ regexp.minimum=File does not contain at least {0} matches for pattern ''{1}''.
 illegal.regexp=Line matches the illegal pattern ''{0}''.
 required.regexp=Required pattern ''{0}'' missing in file.
 duplicate.regexp=Found duplicate pattern ''{0}''.
+
+regexp.empty=Empty (null) pattern.

src/test/java/com/puppycrawl/tools/checkstyle/checks/regexp/RegexpMultilineCheckTest.java

@@ -31,6 +31,7 @@ import org.junit.rules.TemporaryFolder;
 import java.io.File;
 
 import static com.puppycrawl.tools.checkstyle.checks.regexp.MultilineDetector.REGEXP_EXCEEDED;
+import static com.puppycrawl.tools.checkstyle.checks.regexp.MultilineDetector.EMPTY;
 
 public class RegexpMultilineCheckTest extends BaseFileSetCheckTestSupport {
     @Rule public TemporaryFolder temporaryFolder = new TemporaryFolder();
@@ -119,4 +120,13 @@ public class RegexpMultilineCheckTest extends BaseFileSetCheckTestSupport {
         };
         verify(checkConfig, getPath("InputSemantic.java"), expected);
     }
+
+    @Test
+    public void testEmptyFormat() throws Exception {
+        checkConfig.addAttribute("format", null);
+        final String[] expected = {
+            "0: " + getCheckMessage(EMPTY),
+        };
+        verify(checkConfig, getPath("InputSemantic.java"), expected);
+    }
 }