Use same pattern in helper methods.

src/Workspaces/Core/Portable/EmbeddedLanguages/RegularExpressions/RegexLexer.cs

@@ -280,11 +280,6 @@ private static bool IsEscapeCategoryChar(VirtualChar ch)
                 return null;
             }
 
-            if (!IsDecimalDigit(this.CurrentChar))
-            {
-                return null;
-            }
-
             const int MaxValueDiv10 = int.MaxValue / 10;
             const int MaxValueMod10 = int.MaxValue % 10;
 
@@ -308,6 +303,11 @@ private static bool IsEscapeCategoryChar(VirtualChar ch)
                 }
             }
 
+            if (Position == start)
+            {
+                return null;
+            }
+
             var token = CreateToken(RegexKind.NumberToken, ImmutableArray<RegexTrivia>.Empty, GetSubPatternToCurrentPos(start));
             token = token.With(value: value);