diff --git a/src/NzbDrone.Core.Test/CustomFormat/QualityTagFixture.cs b/src/NzbDrone.Core.Test/CustomFormat/QualityTagFixture.cs
index 7c35aeb87..1e07a8230 100644
--- a/src/NzbDrone.Core.Test/CustomFormat/QualityTagFixture.cs
+++ b/src/NzbDrone.Core.Test/CustomFormat/QualityTagFixture.cs
@@ -35,6 +35,7 @@ public class QualityTagFixture : CoreTest
         [TestCase("G_10<>20", TagType.Size, new[] { 10737418240L, 21474836480L})]
         [TestCase("G_15.55<>20", TagType.Size, new[] { 16696685363L, 21474836480L})]
         [TestCase("G_15.55<>25.1908754", TagType.Size, new[] { 16696685363L, 27048496500L})]
+        [TestCase("R__1080", TagType.Resolution, Resolution.R1080P)]
         public void should_parse_tag_from_string(string raw, TagType type, object value, params TagModifier[] modifiers)
         {
             var parsed = new FormatTag(raw);
diff --git a/src/NzbDrone.Core.Test/Datastore/Migration/149_regex_required_tagsFixture.cs b/src/NzbDrone.Core.Test/Datastore/Migration/149_regex_required_tagsFixture.cs
index 998f3dcb9..e1a3c1774 100644
--- a/src/NzbDrone.Core.Test/Datastore/Migration/149_regex_required_tagsFixture.cs
+++ b/src/NzbDrone.Core.Test/Datastore/Migration/149_regex_required_tagsFixture.cs
@@ -32,6 +32,7 @@ public void AddCustomFormat(convert_regex_required_tags c, string name, params s
         [TestCase("C_RE_RERN", "C_RQ_RERN")]
         [TestCase("E_NRER_Director", "E_NRXRQ_Director")]
         [TestCase("G_N_1000<>1000", "G_N_1000<>1000")]
+        [TestCase("G_1000<>1000", "G_1000<>1000")]
         public void should_correctly_convert_format_tag(string original, string converted)
         {
             var db = WithMigrationTestDb(c => { AddCustomFormat(c, "TestFormat", original); });
diff --git a/src/NzbDrone.Core.Test/Datastore/Migration/150_fix_format_tags_double_underscoreFixture.cs b/src/NzbDrone.Core.Test/Datastore/Migration/150_fix_format_tags_double_underscoreFixture.cs
new file mode 100644
index 000000000..f329f18a6
--- /dev/null
+++ b/src/NzbDrone.Core.Test/Datastore/Migration/150_fix_format_tags_double_underscoreFixture.cs
@@ -0,0 +1,57 @@
+using System;
+using System.Linq;
+using System.Collections.Generic;
+using FluentAssertions;
+using Newtonsoft.Json;
+using NUnit.Framework;
+using NzbDrone.Common.Extensions;
+using NzbDrone.Common.Serializer;
+using NzbDrone.Core.Datastore.Migration;
+using NzbDrone.Core.Parser;
+using NzbDrone.Core.Qualities;
+using NzbDrone.Core.Test.Framework;
+
+namespace NzbDrone.Core.Test.Datastore.Migration
+{
+    [TestFixture]
+    public class fix_format_tags_double_underscoreFixture : MigrationTest<fix_format_tags_double_underscore>
+    {
+
+        public void AddCustomFormat(fix_format_tags_double_underscore c, string name, params string[] formatTags)
+        {
+            var customFormat = new {Name = name, FormatTags = formatTags.ToList().ToJson()};
+
+            c.Insert.IntoTable("CustomFormats").Row(customFormat);
+        }
+
+        [TestCase("C_HDR", "C_HDR")]
+        [TestCase("C__HDR", "C_HDR")]
+        [TestCase("C_RXRQ_HDR", "C_RXRQ_HDR")]
+        [TestCase("C_RENR_HDR", "C_RENR_HDR")]
+        [TestCase("E__Director", "E_Director")]
+        [TestCase("G_N_1000<>1000", "G_N_1000<>1000")]
+        [TestCase("G_1000<>1000", "G_1000<>1000")]
+        public void should_correctly_convert_format_tag(string original, string converted)
+        {
+            var db = WithMigrationTestDb(c => { AddCustomFormat(c, "TestFormat", original); });
+
+            var items = QueryItems(db);
+
+            var convertedTags = items.First().DeserializedTags;
+
+            convertedTags.Should().HaveCount(1);
+            convertedTags.First().ShouldBeEquivalentTo(converted);
+        }
+
+        private List<regex_required_tagsFixture.CustomFormatTest149> QueryItems(IDirectDataMapper db)
+        {
+            var items = db.Query<regex_required_tagsFixture.CustomFormatTest149>("SELECT Name, FormatTags FROM CustomFormats");
+
+            return items.Select(i =>
+            {
+                i.DeserializedTags = JsonConvert.DeserializeObject<List<string>>(i.FormatTags);
+                return i;
+            }).ToList();
+        }
+    }
+}
diff --git a/src/NzbDrone.Core.Test/NzbDrone.Core.Test.csproj b/src/NzbDrone.Core.Test/NzbDrone.Core.Test.csproj
index b295ba044..bfd83ac26 100644
--- a/src/NzbDrone.Core.Test/NzbDrone.Core.Test.csproj
+++ b/src/NzbDrone.Core.Test/NzbDrone.Core.Test.csproj
@@ -140,6 +140,7 @@
     <Compile Include="Datastore\Migration\090_update_kickass_urlFixture.cs" />
     <Compile Include="Datastore\Migration\147_custom_formatsFixture.cs" />
     <Compile Include="Datastore\Migration\149_regex_required_tagsFixture.cs" />
+    <Compile Include="Datastore\Migration\150_fix_format_tags_double_underscoreFixture.cs" />
     <Compile Include="Datastore\ObjectDatabaseFixture.cs" />
     <Compile Include="Datastore\PagingSpecExtensionsTests\PagingOffsetFixture.cs" />
     <Compile Include="Datastore\PagingSpecExtensionsTests\ToSortDirectionFixture.cs" />
diff --git a/src/NzbDrone.Core/CustomFormats/FormatTag.cs b/src/NzbDrone.Core/CustomFormats/FormatTag.cs
index e1a6a8ab6..0297231b1 100644
--- a/src/NzbDrone.Core/CustomFormats/FormatTag.cs
+++ b/src/NzbDrone.Core/CustomFormats/FormatTag.cs
@@ -15,7 +15,7 @@ public class FormatTag
         public TagModifier TagModifier { get; set; }
         public object Value { get; set; }
 
-        public static Regex QualityTagRegex = new Regex(@"^(?<type>R|S|M|E|L|C|I|G)(_((?<m_r>RX)|(?<m_re>RQ)|(?<m_n>N)){1,3})?_(?<value>.*)$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
+        public static Regex QualityTagRegex = new Regex(@"^(?<type>R|S|M|E|L|C|I|G)(_((?<m_r>RX)|(?<m_re>RQ)|(?<m_n>N)){0,3})?_(?<value>.*)$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
 
         public static Regex SizeTagRegex = new Regex(@"(?<min>\d+(\.\d+)?)\s*<>\s*(?<max>\d+(\.\d+)?)", RegexOptions.Compiled | RegexOptions.IgnoreCase);
 
diff --git a/src/NzbDrone.Core/Datastore/Migration/149_convert_regex_required_tags.cs b/src/NzbDrone.Core/Datastore/Migration/149_convert_regex_required_tags.cs
index be3350b87..ab006cb91 100644
--- a/src/NzbDrone.Core/Datastore/Migration/149_convert_regex_required_tags.cs
+++ b/src/NzbDrone.Core/Datastore/Migration/149_convert_regex_required_tags.cs
@@ -28,7 +28,8 @@ private void ConvertExistingFormatTags(IDbConnection conn, IDbTransaction tran)
                 if (match.Groups["m_n"].Success) modifiers += "N";
                 if (match.Groups["m_r"].Success) modifiers += "RX";
                 if (match.Groups["m_re"].Success) modifiers += "RQ";
-                return $"{match.Groups["type"].Value}_{modifiers}_{match.Groups["value"].Value}";
+                if (modifiers != "") modifiers = "_" + modifiers;
+                return $"{match.Groups["type"].Value}{modifiers}_{match.Groups["value"].Value}";
             });
 
             updater.Commit();
diff --git a/src/NzbDrone.Core/Datastore/Migration/150_fix_format_tags_double_underscore.cs b/src/NzbDrone.Core/Datastore/Migration/150_fix_format_tags_double_underscore.cs
new file mode 100644
index 000000000..31d0238ad
--- /dev/null
+++ b/src/NzbDrone.Core/Datastore/Migration/150_fix_format_tags_double_underscore.cs
@@ -0,0 +1,33 @@
+using System.Collections.Generic;
+using System.Data;
+using System.Linq;
+using System.Text.RegularExpressions;
+using FluentMigrator;
+using NzbDrone.Common.Serializer;
+using NzbDrone.Core.Datastore.Migration.Framework;
+
+namespace NzbDrone.Core.Datastore.Migration
+{
+    [Migration(150)]
+    public class fix_format_tags_double_underscore : NzbDroneMigrationBase
+    {
+        public static Regex DoubleUnderscore = new Regex(@"^(?<type>R|S|M|E|L|C|I|G)__(?<value>.*)$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
+
+        protected override void MainDbUpgrade()
+        {
+            Execute.WithConnection(ConvertExistingFormatTags);
+        }
+
+        private void ConvertExistingFormatTags(IDbConnection conn, IDbTransaction tran)
+        {
+            var updater = new CustomFormatUpdater149(conn, tran);
+
+            updater.ReplaceInTags(DoubleUnderscore, match =>
+            {
+                return $"{match.Groups["type"].Value}_{match.Groups["value"].Value}";
+            });
+
+            updater.Commit();
+        }
+    }
+}
diff --git a/src/NzbDrone.Core/NzbDrone.Core.csproj b/src/NzbDrone.Core/NzbDrone.Core.csproj
index 5466109d8..866ba4b6a 100644
--- a/src/NzbDrone.Core/NzbDrone.Core.csproj
+++ b/src/NzbDrone.Core/NzbDrone.Core.csproj
@@ -145,6 +145,7 @@
     <Compile Include="Datastore\Migration\145_banner_to_fanart.cs" />
     <Compile Include="Datastore\Migration\144_add_cookies_to_indexer_status.cs" />
     <Compile Include="Datastore\Migration\149_convert_regex_required_tags.cs" />
+    <Compile Include="Datastore\Migration\150_fix_format_tags_double_underscore.cs" />
     <Compile Include="DecisionEngine\Specifications\CustomFormatAllowedByProfileSpecification.cs" />
     <Compile Include="DecisionEngine\Specifications\MaximumSizeSpecification.cs" />
     <Compile Include="DecisionEngine\Specifications\RequiredIndexerFlagsSpecification.cs" />