Allow square bracket expansion when specifying nameservers

I'm finally fed up enough with all the nameserver changes we've had to make on our
self-allocated domains to improve the command.  Now you can simply run:

$ nomulus ... update_domain ... -n ns[1-4].foo.bar

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205282317

java/google/registry/tools/params/NameserversParameter.java

@@ -0,0 +1,80 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.tools.params;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
+import static java.lang.Integer.parseInt;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Splitter;
+import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.google.re2j.Matcher;
+import com.google.re2j.Pattern;
+import java.util.Set;
+import java.util.stream.Stream;
+
+/**
+ * Nameservers CLI parameter converter/validator.
+ *
+ * <p>This accepts a String containing a comma-delimited list of nameservers. Square bracket
+ * notation can be used to expand multiple nameservers, e.g. "ns[1-2].googledomains.com" will be
+ * expanded to "ns1.googledomains.com,ns2.googledomains.com".
+ */
+public final class NameserversParameter extends ParameterConverterValidator<Set<String>> {
+
+  private static final Pattern FORMAT_BRACKETS = Pattern.compile("^(.+)\\[(\\d+)-(\\d+)\\](.+)$");
+
+  public NameserversParameter() {
+    super(
+        "Must be a comma-delimited list of nameservers, "
+            + "optionally using square bracket expansion e.g. foo[1-4].bar.baz notation");
+  }
+
+  @Override
+  public Set<String> convert(String value) {
+    if (Strings.isNullOrEmpty(value)) {
+      return ImmutableSet.of();
+    }
+    return Splitter.on(',')
+        .trimResults()
+        .omitEmptyStrings()
+        .splitToList(value)
+        .stream()
+        .flatMap(NameserversParameter::splitNameservers)
+        .collect(toImmutableSet());
+  }
+
+  @VisibleForTesting
+  static Stream<String> splitNameservers(String ns) {
+    Matcher matcher = FORMAT_BRACKETS.matcher(ns);
+    if (!matcher.matches()) {
+      checkArgument(
+          !ns.contains("[") && !ns.contains("]"), "Could not parse square brackets in %s", ns);
+      return ImmutableList.of(ns).stream();
+    }
+
+    ImmutableList.Builder<String> nameservers = new ImmutableList.Builder<>();
+    int start = parseInt(matcher.group(2));
+    int end = parseInt(matcher.group(3));
+    checkArgument(start <= end, "Number range [%s-%s] is invalid", start, end);
+    for (int i = start; i <= end; i++) {
+      nameservers.add(String.format("%s%d%s", matcher.group(1), i, matcher.group(4)));
+    }
+    return nameservers.build().stream();
+  }
+}