diff --git a/app/logical/tag_name_validator.rb b/app/logical/tag_name_validator.rb
index d97351e34..ec7fe3c07 100644
--- a/app/logical/tag_name_validator.rb
+++ b/app/logical/tag_name_validator.rb
@@ -21,7 +21,7 @@ class TagNameValidator < ActiveModel::EachValidator
       record.errors[attribute] << "'#{value}' cannot contain non-printable characters"
     when /[^[[:ascii:]]]/
       record.errors[attribute] << "'#{value}' must consist of only ASCII characters"
-    when /\A(#{Regexp.union(Tag::METATAGS)}):(.+)\z/i
+    when /\A(#{Tag::METATAGS.join("|")}):(.+)\z/i
       record.errors[attribute] << "'#{value}' cannot begin with '#{$1}:'"
     when /\A(#{Tag.categories.regexp}):(.+)\z/i
       record.errors[attribute] << "'#{value}' cannot begin with '#{$1}:'"
diff --git a/app/models/tag.rb b/app/models/tag.rb
index 4f4540aba..2d3ad47c3 100644
--- a/app/models/tag.rb
+++ b/app/models/tag.rb
@@ -438,7 +438,7 @@ class Tag < ApplicationRecord
     end
 
     def is_metatag?(tag)
-      tag.match?(/\A(#{Regexp.union(METATAGS)}):(.+)\z/i)
+      has_metatag?(tag, *METATAGS)
     end
 
     def is_negated_tag?(tag)
@@ -457,7 +457,7 @@ class Tag < ApplicationRecord
       return nil if tags.blank?
 
       tags = scan_query(tags.to_str) if tags.respond_to?(:to_str)
-      tags.grep(/\A#{Regexp.union(metatags.map(&:to_s))}:(.+)\z/i) { $1 }.first
+      tags.grep(/\A(?:#{metatags.map(&:to_s).join("|")}):(.+)\z/i) { $1 }.first
     end
 
     def parse_query(query, options = {})
@@ -474,7 +474,7 @@ class Tag < ApplicationRecord
       scan_query(query).each do |token|
         q[:tag_count] += 1 unless Danbooru.config.is_unlimited_tag?(token)
 
-        if token =~ /\A(#{Regexp.union(METATAGS)}):(.+)\z/i
+        if token =~ /\A(#{METATAGS.join("|")}):(.+)\z/i
           g1 = $1.downcase
           g2 = $2
           case g1
diff --git a/test/unit/tag_test.rb b/test/unit/tag_test.rb
index 0cffd4843..81f71de40 100644
--- a/test/unit/tag_test.rb
+++ b/test/unit/tag_test.rb
@@ -138,6 +138,7 @@ class TagTest < ActiveSupport::TestCase
       assert_equal([:lte, 2], Tag.parse_query("id:..2")[:post_id])
       assert_equal([:gt, 2], Tag.parse_query("id:>2")[:post_id])
       assert_equal([:lt, 3], Tag.parse_query("id:<3")[:post_id])
+      assert_equal([:lt, 3], Tag.parse_query("ID:<3")[:post_id])
 
       Tag.expects(:normalize_tags_in_query).returns(nil)
       assert_equal(["acb"], Tag.parse_query("a*b")[:tags][:include])
@@ -160,6 +161,7 @@ class TagTest < ActiveSupport::TestCase
       assert_equal(false, Tag.is_simple_tag?("~foo"))
       assert_equal(false, Tag.is_simple_tag?("foo*"))
       assert_equal(false, Tag.is_simple_tag?("fav:1234"))
+      assert_equal(false, Tag.is_simple_tag?("FAV:1234"))
       assert_equal(false, Tag.is_simple_tag?("pool:1234"))
       assert_equal(false, Tag.is_simple_tag?('source:"foo bar baz"'))
       assert_equal(false, Tag.is_simple_tag?("foo bar"))
@@ -241,6 +243,7 @@ class TagTest < ActiveSupport::TestCase
       should_not allow_value("foo\abar").for(:name).on(:create)
       should_not allow_value("café").for(:name).on(:create)
       should_not allow_value("東方").for(:name).on(:create)
+      should_not allow_value("FAV:blah").for(:name).on(:create)
 
       metatags = Tag::METATAGS + TagCategory.mapping.keys
       metatags.each do |metatag|