diff --git a/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.cs b/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.cs
index 846fcbce60fa65..8fa496d6d7bdb5 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.cs
+++ b/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.cs
@@ -714,11 +714,12 @@ private static void Parse(string pattern, RegexOptions options, RegexParseError?
             if (error != null)
             {
                 Assert.InRange(offset, 0, int.MaxValue);
-                Throws(error.Value, offset, () => new Regex(pattern, options));
+                Throws(pattern, options, error.Value, offset, () => new Regex(pattern, options));
                 return;
             }
 
             Assert.Equal(-1, offset);
+            LogActual(pattern, options, RegexParseError.Unknown, -1);
 
             // Nothing to assert here without having access to internals.
             new Regex(pattern, options); // Does not throw
@@ -726,6 +727,27 @@ private static void Parse(string pattern, RegexOptions options, RegexParseError?
             ParsePatternFragments(pattern, options);
         }
 
+        private static void LogActual(string pattern, RegexOptions options, RegexParseError error, int offset)
+        {   
+            // To conveniently add new interesting patterns to these tests, add them to the code in the format:
+            //
+            // [InlineData("SOMEREGEX1", RegexOptions.None, null)]
+            // [InlineData("SOMEREGEX2", RegexOptions.None, null)]
+            // ...
+            //
+            // then uncomment the lines below, and the correct baseline will be written to the file, eg
+            //
+            // [InlineData(@"SOMEREGEX1", RegexOptions.None, RegexParseError.UnrecognizedEscape, 3)]
+            // [InlineData(@"SOMEREGEX2", RegexOptions.None, InsufficientClosingParentheses, 2)]
+            // ...
+            // 
+            //string s = (error == RegexParseError.Unknown) ?
+            //    @$"        [InlineData(@""{pattern}"", RegexOptions.{options.ToString()}, null)]" :
+            //    @$"        [InlineData(@""{pattern}"", RegexOptions.{options.ToString()}, RegexParseError.{error.ToString()}, {offset})]";
+
+            // File.AppendAllText(@"/tmp/out.cs", s + "\n");
+        }
+
         private static void ParsePatternFragments(string pattern, RegexOptions options)
         {
             // Trim the input in various places and parse.
@@ -755,7 +777,7 @@ private static void ParsePatternFragments(string pattern, RegexOptions options)
         /// </summary>
         /// <param name="error">The expected parse error</param>
         /// <param name="action">The action to invoke.</param>
-        static partial void Throws(RegexParseError error, int offset, Action action);
+        static partial void Throws(string pattern, RegexOptions options, RegexParseError error, int offset, Action action);
 
         /// <summary>
         /// Checks that action succeeds or throws either a RegexParseException or an ArgumentException depending on the
diff --git a/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netcoreapp.cs b/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netcoreapp.cs
index cc1d5f8658fe9b..13ff23b7dde4d6 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netcoreapp.cs
+++ b/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netcoreapp.cs
@@ -110,7 +110,7 @@ public partial class RegexParserTests
         [InlineData(@"(?P<a>.)(?P<a>.)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 3)]
         [InlineData(@"[a-\A]", RegexOptions.None, RegexParseError.UnrecognizedEscape, 5)]
         [InlineData(@"[a-\z]", RegexOptions.None, RegexParseError.UnrecognizedEscape, 5)]
-        [InlineData(@"[a-\b]", RegexOptions.None, RegexParseError.ReversedCharacterRange, 5)]
+        [InlineData(@"[a-\b]", RegexOptions.None, RegexParseError.ReversedCharacterRange, 5)] // Nim: not an error
         [InlineData(@"[a-\-]", RegexOptions.None, RegexParseError.ReversedCharacterRange, 5)]
         [InlineData(@"[a-\-b]", RegexOptions.None, RegexParseError.ReversedCharacterRange, 5)]
         [InlineData(@"[a-\-\-b]", RegexOptions.None, RegexParseError.ReversedCharacterRange, 5)]
@@ -127,6 +127,115 @@ public partial class RegexParserTests
         [InlineData(@"[a-[:lower:]]", RegexOptions.None, null)] // errors in rust: range_end_no_class
         // End of Rust parser tests ==============
 
+        // Following are borrowed from Nim tests
+        // https://github.com/nitely/nim-regex/blob/eeefb4f51264ff3bc3b36caf55672a74f52f5ef5/tests/tests.nim
+        [InlineData(@"?", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"?|?", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"?abc", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"(?P<abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_>abc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 3)] // Nim: not an error
+        [InlineData(@"(?Pabc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 3)]
+        [InlineData(@"(?u-q)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 3)]
+        [InlineData(@"(?uq)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 3)]
+        [InlineData(@"(\b)", RegexOptions.None, null)]
+        [InlineData(@"(+)", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 2)]
+        [InlineData(@"(a)b)", RegexOptions.None, RegexParseError.InsufficientOpeningParentheses, 5)]
+        [InlineData(@"(b(a)", RegexOptions.None, RegexParseError.InsufficientClosingParentheses, 5)]
+        [InlineData(@"[-", RegexOptions.None, RegexParseError.UnterminatedBracket, 2)]
+        [InlineData(@"[-a", RegexOptions.None, RegexParseError.UnterminatedBracket, 3)]
+        [InlineData(@"[[:abc:]]", RegexOptions.None, null)] // Nim: "Invalid ascii set. `abc` is not a valid name"
+        [InlineData(@"[[:alnum:", RegexOptions.None, RegexParseError.UnterminatedBracket, 9)]
+        [InlineData(@"[[:alnum]]", RegexOptions.None, null)] // Nim: "Invalid ascii set. Expected [:name:]"
+        [InlineData(@"[]", RegexOptions.None, RegexParseError.UnterminatedBracket, 2)]
+        [InlineData(@"[]a", RegexOptions.None, RegexParseError.UnterminatedBracket, 3)]
+        [InlineData(@"[]abc", RegexOptions.None, RegexParseError.UnterminatedBracket, 5)]
+        [InlineData(@"[\\", RegexOptions.None, RegexParseError.UnterminatedBracket, 3)]
+        [InlineData(@"[^]", RegexOptions.None, RegexParseError.UnterminatedBracket, 3)]
+        [InlineData(@"[a-", RegexOptions.None, RegexParseError.UnterminatedBracket, 3)]
+        [InlineData(@"[a-\w]", RegexOptions.None, RegexParseError.ShorthandClassInCharacterRange, 5)]
+        [InlineData(@"[a", RegexOptions.None, RegexParseError.UnterminatedBracket, 2)]
+        [InlineData(@"[abc", RegexOptions.None, RegexParseError.UnterminatedBracket, 4)]
+        [InlineData(@"[d-c]", RegexOptions.None, RegexParseError.ReversedCharacterRange, 4)]
+        [InlineData(@"[z-[:alnum:]]", RegexOptions.None, null)] // Nim: "Invalid set range. Start must be lesser than end"
+        [InlineData(@"{10}", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"*abc", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"\12", RegexOptions.None, null)] // Nim: "Invalid octal literal. Expected 3 octal digits, but found 2"
+        [InlineData(@"\12@", RegexOptions.None, null)] // Nim: "Invalid octal literal. Expected octal digit, but found @"
+        [InlineData(@"\b?", RegexOptions.None, null)]
+        [InlineData(@"\b*", RegexOptions.None, null)]
+        [InlineData(@"\b+", RegexOptions.None, null)]
+        [InlineData(@"\p{11", RegexOptions.None, RegexParseError.InvalidUnicodePropertyEscape, 5)]
+        [InlineData(@"\p{11}", RegexOptions.None, RegexParseError.UnrecognizedUnicodeProperty, 6)]
+        [InlineData(@"\p{Bb}", RegexOptions.None, RegexParseError.UnrecognizedUnicodeProperty, 6)]
+        [InlineData(@"\p11", RegexOptions.None, RegexParseError.InvalidUnicodePropertyEscape, 2)]
+        [InlineData(@"\pB", RegexOptions.None, RegexParseError.InvalidUnicodePropertyEscape, 2)]
+        [InlineData(@"\u123", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 2)]
+        [InlineData(@"\U123", RegexOptions.None, RegexParseError.UnrecognizedEscape, 2)]
+        [InlineData(@"\U123@a", RegexOptions.None, RegexParseError.UnrecognizedEscape, 2)]
+        [InlineData(@"\u123@abc", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 6)]
+        [InlineData(@"\UFFFFFFFF", RegexOptions.None, RegexParseError.UnrecognizedEscape, 2)]
+        [InlineData(@"\x{00000000A}", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 3)]
+        [InlineData(@"\x{2f894", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 3)]
+        [InlineData(@"\x{61@}", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 3)]
+        [InlineData(@"\x{7fffffff}", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 3)] // Nim: not an error (supports Unicode beyond basic multilingual plane)
+        [InlineData(@"\x{FFFFFFFF}", RegexOptions.None, RegexParseError.InsufficientOrInvalidHexDigits, 3)]
+        [InlineData(@"+", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"+abc", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 1)]
+        [InlineData(@"a???", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 4)]
+        [InlineData(@"a??*", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 4)]
+        [InlineData(@"a??+", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 4)]
+        [InlineData(@"a?*", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a?+", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a(?P<>abc)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 4)]
+        [InlineData(@"a(?P<asd)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 4)]
+        [InlineData(@"a{,}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{,1}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{0,101}", RegexOptions.None, null)] // Nim error: "Invalid repetition range. Expected 100 repetitions or less, but found: 101"
+        [InlineData(@"a{0,a}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{0,bad}", RegexOptions.None, null)] // Nim error: "Invalid repetition range. Range can only contain digits"
+        [InlineData(@"a{1,,,2}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{1,,}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{1,,2}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{1,", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{1,}??", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 7)]
+        [InlineData(@"a{1,}?", RegexOptions.None, null)]
+        [InlineData(@"a{1,}*", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 6)]
+        [InlineData(@"a{1,}+", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 6)]
+        [InlineData(@"a{1,101}", RegexOptions.None, null)]
+        [InlineData(@"a{1,x}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{1", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a{1}??", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 6)]
+        [InlineData(@"a{1}?", RegexOptions.None, null)]
+        [InlineData(@"a{1}*", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 5)]
+        [InlineData(@"a{1}+", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 5)]
+        [InlineData(@"a{1111111111}", RegexOptions.None, null)] // Nim error: "Invalid repetition range. Max value is 32767."
+        [InlineData(@"a{1x}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a*??", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 4)]
+        [InlineData(@"a*{,}", RegexOptions.None, null)] // Nim error
+        [InlineData(@"a*{0}", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a*{1}", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a**", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a*****", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a*+", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a+??", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 4)]
+        [InlineData(@"a+*", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a++", RegexOptions.None, RegexParseError.NestedQuantifiersNotParenthesized, 3)]
+        [InlineData(@"a|?", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 3)]
+        [InlineData(@"a|?b", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 3)]
+        [InlineData(@"a|*", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 3)]
+        [InlineData(@"a|*b", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 3)]
+        [InlineData(@"a|+", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 3)]
+        [InlineData(@"a|+b", RegexOptions.None, RegexParseError.QuantifierAfterNothing, 3)]
+        [InlineData(@"aaa(?Pabc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 6)]
+        [InlineData(@"abc(?P<abc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 6)]
+        [InlineData(@"abc(?Pabc)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 6)]
+        [InlineData(@"abc(?q)", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 6)]
+        [InlineData(@"abc[]", RegexOptions.None, RegexParseError.UnterminatedBracket, 5)]
+        [InlineData(@"abc\A{10}", RegexOptions.None, null)] // Nim error:  "Invalid repetition range, either char, shorthand (i.e: \\w), group, or set expected before repetition range"
+        [InlineData(@"\uD87E\uDC94(?Pabc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 15)]
+        [InlineData(@"\uD87E\uDC94aaa(?Pabc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 18)]
+        [InlineData(@"\uD87E\uDC94\uD87E\uDC94\uD87E\uDC94(?Pabc", RegexOptions.None, RegexParseError.InvalidGroupingConstruct, 39)]
+        // End of Nim parser tests ==============
+
         [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
         public void Parse_Netcoreapp(string pattern, RegexOptions options, RegexParseError? error, int offset = -1)
         {
@@ -157,7 +266,7 @@ public void RegexParseException_Serializes()
         /// </summary>
         /// <param name="error">The expected parse error</param>
         /// <param name="action">The action to invoke.</param>
-        static partial void Throws(RegexParseError error, int offset, Action action)
+        static partial void Throws(string pattern, RegexOptions options, RegexParseError error, int offset, Action action)
         {
             try
             {
@@ -171,16 +280,19 @@ static partial void Throws(RegexParseError error, int offset, Action action)
                 if (error == regexParseError)
                 {
                     Assert.Equal(offset, e.Offset);
+                    LogActual(pattern, options, regexParseError, e.Offset);
                     return;
                 }
 
+                LogActual(pattern, options, regexParseError, e.Offset);
                 throw new XunitException($"Expected RegexParseException with error {error} offset {offset} -> Actual error: {regexParseError} offset {e.Offset})");
             }
             catch (Exception e)
             {
-                throw new XunitException($"Expected RegexParseException -> Actual: ({e})");
+                throw new XunitException($"Expected RegexParseException for pattern '{pattern}' -> Actual: ({e})");
             }
 
+            LogActual(pattern, options, RegexParseError.Unknown, -1);
             throw new XunitException($"Expected RegexParseException with error: ({error}) -> Actual: No exception thrown");
         }
 
diff --git a/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netfx.cs b/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netfx.cs
index e70d29a8dd388f..b7b3d56505a3ca 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netfx.cs
+++ b/src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netfx.cs
@@ -16,7 +16,7 @@ public partial class RegexParserTests
         /// </summary>
         /// <param name="error">The expected parse error</param>
         /// <param name="action">The action to invoke.</param>
-        static partial void Throws(RegexParseError error, int offset, Action action)
+        static partial void Throws(string pattern, RegexOptions options, RegexParseError error, int offset, Action action)
         {
             try
             {
diff --git a/src/libraries/System.Text.RegularExpressions/tests/THIRD-PARTY-NOTICES.TXT b/src/libraries/System.Text.RegularExpressions/tests/THIRD-PARTY-NOTICES.TXT
index 8c51928cfb1e72..b5f4599ed3a336 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/THIRD-PARTY-NOTICES.TXT
+++ b/src/libraries/System.Text.RegularExpressions/tests/THIRD-PARTY-NOTICES.TXT
@@ -35,4 +35,29 @@ SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
 IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
-DEALINGS IN THE SOFTWARE.
\ No newline at end of file
+DEALINGS IN THE SOFTWARE.
+
+License notice for https://github.com/nitely/nim-regex
+-------------------------------
+
+MIT License
+
+Copyright (c) 2017 Esteban Castro Borsani
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
\ No newline at end of file