CSHARP-3297: Handle incomplete regex patterns in JsonScanner

Author: rkyser-avasure

src/MongoDB.Bson/IO/JsonScanner.cs

@@ -369,6 +369,7 @@ private static JsonToken GetRegularExpressionToken(JsonBuffer buffer)
                         {
                             case '/': state = RegularExpressionState.InOptions; break;
                             case '\\': state = RegularExpressionState.InEscapeSequence; break;
+                            case -1: state = RegularExpressionState.Invalid; break;
                             default: state = RegularExpressionState.InPattern; break;
                         }
                         break;

tests/MongoDB.Bson.Tests/IO/JsonScannerTests.cs

@@ -13,6 +13,7 @@
 * limitations under the License.
 */
 
+using System;
 using MongoDB.Bson.IO;
 using Xunit;
 
@@ -405,6 +406,20 @@ public void TestMinusZeroExponentMinusTwelve()
             Assert.Equal(',', buffer.Read());
         }
 
+        [Theory]
+        [InlineData("/")]
+        [InlineData("/pattern")]
+        [InlineData("/pattern\\/")]
+        public void TestMissingClosingSlash(string jsonRegex)
+        {
+            var buffer = new JsonBuffer(jsonRegex);
+
+            var exception = Record.Exception(() => JsonScanner.GetNextToken(buffer));
+
+            Assert.IsType<FormatException>(exception);
+            Assert.StartsWith("Invalid JSON regular expression", exception.Message);
+        }
+
         [Fact]
         public void TestRegularExpressionEmpty()
         {