Validate that cloud name is valid label value.

src/main/java/cloud/dnation/jenkins/plugins/hetzner/Helper.java

@@ -51,12 +51,14 @@
 import java.util.logging.Level;
 import java.util.logging.LogRecord;
 import java.util.logging.SimpleFormatter;
+import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
 import static cloud.dnation.jenkins.plugins.hetzner.HetznerConstants.SHUTDOWN_TIME_BUFFER;
 
 @UtilityClass
 public class Helper {
+    private static final Pattern LABEL_VALUE_RE = Pattern.compile("^(?![0-9]+$)(?!-)[a-zA-Z0-9-_.]{0,63}(?<!-)$");
     private static final String SSH_RSA = "ssh-rsa";
 
     /**
@@ -202,4 +204,11 @@ public static List<HetznerServerAgent> getHetznerAgents() {
                 .map(HetznerServerAgent.class::cast)
                 .collect(Collectors.toList());
     }
+
+    public static boolean isValidLabelValue(String value) {
+        if (Strings.isNullOrEmpty(value)) {
+            return false;
+        }
+        return LABEL_VALUE_RE.matcher(value).matches();
+    }
 }

src/main/java/cloud/dnation/jenkins/plugins/hetzner/HetznerCloud.java

@@ -207,6 +207,15 @@
             }
         }
 
+        @Restricted(NoExternalUse.class)
+        @RequirePOST
+        public FormValidation doCheckCloudName(@QueryParameter String name) {
+            if (Helper.isValidLabelValue(name)) {
+                return FormValidation.ok();
+            }
+            return FormValidation.error("Cloud name is not a valid label value: %s", name);
+        }
+
         @Restricted(NoExternalUse.class)
         @RequirePOST
         public ListBoxModel doFillCredentialsIdItems(@AncestorInPath Item owner) {

src/main/resources/cloud/dnation/jenkins/plugins/hetzner/HetznerCloud/config.jelly

@@ -16,7 +16,7 @@
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form" xmlns:c="/lib/credentials" xmlns:st="jelly:stapler">
     <f:entry title="${%Name}" field="name">
-        <f:textbox default="hetzner"/>
+        <f:textbox default="hetzner" checkUrl="'${rootURL}/descriptorByName/cloud.dnation.jenkins.plugins.hetzner.HetznerCloud/checkCloudName?name='+escape(this.value)" />
     </f:entry>
     <f:entry field="instanceCapStr" title="${%Instance Cap}">
         <f:textbox default="10"/>

src/main/resources/cloud/dnation/jenkins/plugins/hetzner/HetznerCloud/help-name.html

@@ -14,5 +14,9 @@
  limitations under the License.
 -->
 <div>
-    Provide a name for this Hetzner Cloud
+    Provide a name for this Hetzner Cloud.
+    Must be a valid <a href="https://docs.hetzner.cloud/#labels">label value</a>.
+    <p>
+    <i>Valid label values must be a string of 63 characters or less and must be empty or begin and end with an alphanumeric character ([a-z0-9A-Z]) with dashes (-), underscores (_), dots (.), and alphanumerics between.</i>
+    </p>
 </div>

src/test/java/cloud/dnation/jenkins/plugins/hetzner/HelperTest.java

@@ -65,4 +65,13 @@ public void testIsPossiblyLong() {
         assertFalse(Helper.isPossiblyLong("0"));
         assertFalse(Helper.isPossiblyLong("not-a-number"));
     }
+
+    @Test
+    public void testIsValidLabelValue() {
+        assertFalse(Helper.isValidLabelValue(""));
+        assertFalse(Helper.isValidLabelValue(null));
+        assertTrue(Helper.isValidLabelValue("cloud-01"));
+        assertTrue(Helper.isValidLabelValue("cloud_01"));
+        assertFalse(Helper.isValidLabelValue("cloud 01"));
+    }
 }