test: Make the domain name regex match the go connector and add tests. (#2179)

Co-authored-by: kgala2 <[email protected]>

Author: hessjcg

core/src/main/java/com/google/cloud/sql/core/CloudSqlInstanceName.java

@@ -51,7 +51,8 @@ public class CloudSqlInstanceName {
    * $: Matches the end of the string.<br>
    */
   private static final Pattern DOMAIN_NAME =
-      Pattern.compile("^(?=.{1,255}$)(?!-)[A-Za-z0-9-]+(\\.[A-Za-z0-9-]+)*\\.[A-Za-z]{2,}$");
+      Pattern.compile(
+          "^(?:[_a-z0-9](?:[_a-z0-9-]{0,61}[a-z0-9])?\\.)+(?:[a-z](?:[a-z0-9-]{0,61}[a-z0-9])?)?$");
 
   private final String projectId;
   private final String regionId;

core/src/test/java/com/google/cloud/sql/core/CloudSqlInstanceNameTest.java

@@ -60,4 +60,14 @@ public void parseBadConnectionName() {
 
     assertThat(ex).hasMessageThat().contains("Cloud SQL connection name is invalid");
   }
+
+  @Test
+  public void testValidDomains() {
+    assertThat(CloudSqlInstanceName.isValidDomain("prod-db.mycompany.example.com")).isTrue();
+    assertThat(CloudSqlInstanceName.isValidDomain("example.com.")).isTrue();
+    assertThat(CloudSqlInstanceName.isValidDomain("-example.com")).isFalse();
+    assertThat(CloudSqlInstanceName.isValidDomain("example")).isFalse();
+    assertThat(CloudSqlInstanceName.isValidDomain("127.0.0.1")).isFalse();
+    assertThat(CloudSqlInstanceName.isValidDomain("0:0:0:0:0:0:0:1")).isFalse();
+  }
 }