Introduce AssertionTestUtils.assertMessageMatches

assertMessageMatches() is a convenience method for using a RegEx instead
of assertMessageStartsWith()/assertMessageEndsWith() combos.

This commit makes use of the new method in AssertNullAssertionsTests but
leaves all other existing tests unchanged.

assertMessageMatches() can be used for new tests, or we can slowly
migrate to it where it makes sense as we modify existing tests for some
other reason.

junit-jupiter-engine/src/test/java/org/junit/jupiter/api/AssertNullAssertionsTests.java

@@ -11,8 +11,8 @@
 package org.junit.jupiter.api;
 
 import static org.junit.jupiter.api.AssertionTestUtils.assertExpectedAndActualValues;
-import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEndsWith;
-import static org.junit.jupiter.api.AssertionTestUtils.assertMessageStartsWith;
+import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEquals;
+import static org.junit.jupiter.api.AssertionTestUtils.assertMessageMatches;
 import static org.junit.jupiter.api.AssertionTestUtils.expectAssertionFailedError;
 import static org.junit.jupiter.api.Assertions.assertNull;
 
@@ -46,7 +46,7 @@ void assertNullWithNonNullObject() {
 			expectAssertionFailedError();
 		}
 		catch (AssertionFailedError ex) {
-			assertMessageEndsWith(ex, "expected: <null> but was: <foo>");
+			assertMessageEquals(ex, "expected: <null> but was: <foo>");
 			assertExpectedAndActualValues(ex, null, "foo");
 		}
 	}
@@ -76,8 +76,7 @@ private void assertNullWithNonNullObjectWithNullStringReturnedFromToString(Suppl
 			// Should look something like:
 			// expected: <null> but was: java.lang.String@264b3504<null>
 			String prefix = (messageSupplier != null ? messageSupplier.get() + " ==> " : "");
-			assertMessageStartsWith(ex, prefix + "expected: <null> but was: java.lang.String@");
-			assertMessageEndsWith(ex, "<null>");
+			assertMessageMatches(ex, prefix + "expected: <null> but was: java\\.lang\\.String@.+<null>");
 			assertExpectedAndActualValues(ex, null, actual);
 		}
 	}
@@ -107,9 +106,8 @@ private void assertNullWithNonNullObjectWithNullReferenceReturnedFromToString(Su
 			// Should look something like:
 			// expected: <null> but was: org.junit.jupiter.api.AssertNullAssertionsTests$NullToString@4e7912d8<null>
 			String prefix = (messageSupplier != null ? messageSupplier.get() + " ==> " : "");
-			assertMessageStartsWith(ex,
-				prefix + "expected: <null> but was: org.junit.jupiter.api.AssertNullAssertionsTests$NullToString@");
-			assertMessageEndsWith(ex, "<null>");
+			assertMessageMatches(ex, prefix
+					+ "expected: <null> but was: org\\.junit\\.jupiter\\.api\\.AssertNullAssertionsTests\\$NullToString@.+<null>");
 			assertExpectedAndActualValues(ex, null, actual);
 		}
 	}
@@ -121,8 +119,7 @@ void assertNullWithNonNullObjectAndMessage() {
 			expectAssertionFailedError();
 		}
 		catch (AssertionFailedError ex) {
-			assertMessageStartsWith(ex, "a message");
-			assertMessageEndsWith(ex, "expected: <null> but was: <foo>");
+			assertMessageEquals(ex, "a message ==> expected: <null> but was: <foo>");
 			assertExpectedAndActualValues(ex, null, "foo");
 		}
 	}
@@ -134,8 +131,7 @@ void assertNullWithNonNullObjectAndMessageSupplier() {
 			expectAssertionFailedError();
 		}
 		catch (AssertionFailedError ex) {
-			assertMessageStartsWith(ex, "test");
-			assertMessageEndsWith(ex, "expected: <null> but was: <foo>");
+			assertMessageEquals(ex, "test ==> expected: <null> but was: <foo>");
 			assertExpectedAndActualValues(ex, null, "foo");
 		}
 	}

junit-jupiter-engine/src/test/java/org/junit/jupiter/api/AssertionTestUtils.java

@@ -38,6 +38,13 @@ static void assertMessageEquals(Throwable ex, String msg) throws AssertionError
 		}
 	}
 
+	static void assertMessageMatches(Throwable ex, String regex) throws AssertionError {
+		if (!ex.getMessage().matches(regex)) {
+			throw new AssertionError("Exception message should match regular expression [" + regex + "], but was ["
+					+ ex.getMessage() + "].");
+		}
+	}
+
 	static void assertMessageStartsWith(Throwable ex, String msg) throws AssertionError {
 		if (!ex.getMessage().startsWith(msg)) {
 			throw new AssertionError(