diff --git a/include/rapidjson/internal/regex.h b/include/rapidjson/internal/regex.h
index d317daa3a..c20629427 100644
--- a/include/rapidjson/internal/regex.h
+++ b/include/rapidjson/internal/regex.h
@@ -375,14 +375,14 @@ class GenericRegex {
     bool Eval(Stack<Allocator>& operandStack, Operator op) {
         switch (op) {
             case kConcatenation:
-                if (operandStack.GetSize() >= sizeof(Frag) * 2) {
+                RAPIDJSON_ASSERT(operandStack.GetSize() >= sizeof(Frag) * 2);
+                {
                     Frag e2 = *operandStack.template Pop<Frag>(1);
                     Frag e1 = *operandStack.template Pop<Frag>(1);
                     Patch(e1.out, e2.start);
                     *operandStack.template Push<Frag>() = Frag(e1.start, e2.out, Min(e1.minIndex, e2.minIndex));
-                    return true;
                 }
-                return false;
+                return true;
 
             case kAlternation:
                 if (operandStack.GetSize() >= sizeof(Frag) * 2) {
@@ -430,8 +430,7 @@ class GenericRegex {
 
     bool EvalQuantifier(Stack<Allocator>& operandStack, unsigned n, unsigned m) {
         RAPIDJSON_ASSERT(n <= m);
-        if (operandStack.GetSize() < sizeof(Frag))
-            return false;
+        RAPIDJSON_ASSERT(operandStack.GetSize() >= sizeof(Frag));
 
         if (n == 0) {
             if (m == 0)                             // a{0} not support
@@ -647,8 +646,7 @@ class GenericRegex {
 
     // Return whether the added states is a match state
     bool AddState(Stack<Allocator>& l, SizeType index) const {
-        if (index == kRegexInvalidState)
-            return true;
+        RAPIDJSON_ASSERT(index != kRegexInvalidState);
 
         const State& s = GetState(index);
         if (s.out1 != kRegexInvalidState) { // Split
diff --git a/test/unittest/regextest.cpp b/test/unittest/regextest.cpp
index e3371d168..b497df6ab 100644
--- a/test/unittest/regextest.cpp
+++ b/test/unittest/regextest.cpp
@@ -17,6 +17,14 @@
 
 using namespace rapidjson::internal;
 
+TEST(Regex, Single) {
+    Regex re("a");
+    ASSERT_TRUE(re.IsValid());
+    EXPECT_TRUE(re.Match("a"));
+    EXPECT_FALSE(re.Match(""));
+    EXPECT_FALSE(re.Match("b"));
+}
+
 TEST(Regex, Concatenation) {
     Regex re("abc");
     ASSERT_TRUE(re.IsValid());
@@ -560,6 +568,9 @@ TEST(Regex, Invalid) {
     TEST_INVALID("a{1,0}");
     TEST_INVALID("a{-1,0}");
     TEST_INVALID("a{-1,1}");
+    TEST_INVALID("a{4294967296}"); // overflow of unsigned
+    TEST_INVALID("a{1a}");
+    TEST_INVALID("[");
     TEST_INVALID("[]");
     TEST_INVALID("[^]");
     TEST_INVALID("[\\a]");