diff --git a/bigint.h b/bigint.h
index 9a7035a..43725b3 100644
--- a/bigint.h
+++ b/bigint.h
@@ -57,6 +57,11 @@ namespace BigInt {
                     throw std::runtime_error("Invalid Big Integer.");
                 is_neg = true;
             }
+            else if (s[0] == '0' && s[1] == 'x') {
+                // Hex string
+                is_neg = false;
+                vec = hex_to_vector(s);
+            }
             else {
                 is_neg = false;
                 vec = string_to_vector(s);
@@ -381,6 +386,7 @@ namespace BigInt {
         }
 
         static std::vector<long long> string_to_vector(std::string input);
+        static std::vector<long long> hex_to_vector(std::string input);
 
         static std::string vector_to_string(const std::vector<long long>& input);
 
@@ -450,12 +456,26 @@ namespace BigInt {
 
 
     inline bool bigint::is_bigint(const std::string& s) {
-        if (s.empty() || (s.length() > 1 && s[0] == '0'))
+        if (s.empty())
             return false;
 
+        // Check for leading negative sign
         if (s[0] == '-') {
-            return s.find_first_not_of("0123456789", 1) == std::string::npos;
+            if (s.length() == 1) return false;
+            // Recursively validate the rest as a positive bigint
+            // Note: leading zeros check still applies to the magnitude
+            return is_bigint(s.substr(1));
+        }
+
+        // Check for hex string "0x..."
+        if (s.length() > 2 && s[0] == '0' && s[1] == 'x') {
+            return s.find_first_not_of("0123456789abcdefABCDEF", 2) == std::string::npos;
         }
+
+        // Reject leading zeros for decimal numbers
+        if (s.length() > 1 && s[0] == '0')
+            return false;
+
         return s.find_first_not_of("0123456789", 0) == std::string::npos;
     }
 
@@ -844,6 +864,38 @@ namespace BigInt {
         return result;
     }
 
+    inline std::vector<long long> bigint::hex_to_vector(std::string input) {
+        // Strip "0x" prefix
+        const std::string hex = input.substr(2);
+
+        std::vector<long long> result = {0};
+
+        for (const char c : hex) {
+            unsigned long long digit;
+            if (c >= '0' && c <= '9')      digit = c - '0';
+            else if (c >= 'a' && c <= 'f') digit = c - 'a' + 10;
+            else if (c >= 'A' && c <= 'F') digit = c - 'A' + 10;
+            else throw std::runtime_error("Invalid hex character.");
+
+            // Multiply current value by 16 and add digit.
+            // Max intermediate value: (MAX_SIZE-1)*16+15 = 15,999,999,999,999,999,999
+            // which fits in unsigned long long (max ~18.4 * 10^18).
+            unsigned long long carry = digit;
+            for (int i = static_cast<int>(result.size()) - 1; i >= 0; --i) {
+                const unsigned long long val = static_cast<unsigned long long>(result[i]) * 16 + carry;
+                result[i] = static_cast<long long>(val % static_cast<unsigned long long>(MAX_SIZE));
+                carry = val / static_cast<unsigned long long>(MAX_SIZE);
+            }
+            // carry <= 15 < MAX_SIZE, so this loop runs at most once
+            while (carry > 0) {
+                result.insert(result.begin(), static_cast<long long>(carry % static_cast<unsigned long long>(MAX_SIZE)));
+                carry /= static_cast<unsigned long long>(MAX_SIZE);
+            }
+        }
+
+        return result;
+    }
+
     inline std::string bigint::vector_to_string(const std::vector<long long>& input) {
         std::stringstream ss;
         bool first = true;
diff --git a/tests/test.cpp b/tests/test.cpp
index 041a796..2d86402 100644
--- a/tests/test.cpp
+++ b/tests/test.cpp
@@ -99,8 +99,10 @@ class BigInt_ModParamTest : public ::testing::TestWithParam<TestCase>{};
 
 TEST(Test_BigInt, Invalid_Tests) {
     EXPECT_THROW(bigint('a'), std::runtime_error);
+    EXPECT_THROW(bigint('z'), std::runtime_error);
     EXPECT_THROW(bigint(static_cast<char>(0)), std::runtime_error);
     EXPECT_THROW(bigint(static_cast<char>(-1)), std::runtime_error);
+    EXPECT_THROW(bigint(static_cast<char>(99)), std::runtime_error);
     EXPECT_THROW(bigint("a"), std::runtime_error);
     EXPECT_THROW(bigint("?"), std::runtime_error);
     EXPECT_THROW(bigint(""), std::runtime_error);
@@ -121,6 +123,8 @@ TEST(Test_BigInt, Invalid_Tests) {
 
 TEST(Test_BigInt, Creation_Tests) {
     int my_int = 100;
+    unsigned int hex_int = 0xDEADBEEF;
+    int neg_hex_int = -0xBEEF;
     long my_long = 100;
     long long my_longlong = 100;
     long long my_longlong2 = LLONG_MAX;
@@ -131,10 +135,15 @@ TEST(Test_BigInt, Creation_Tests) {
     std::string my_string = "100";
     std::string my_string3 = "-9223372036854775808";
     std::string my_string4 = "-9223372036854775809";
+    std::string hex_string = "0xDEADBEEF0FF1CEF0CACC1A";
+    std::string neg_hex_string = "-0xBEEF";
 
     EXPECT_EQ(bigint(my_int), 100);
     EXPECT_EQ(bigint(-my_int), -100);
 
+    EXPECT_EQ(bigint(hex_int), 3735928559);
+    EXPECT_EQ(bigint(neg_hex_int), -48879);
+
     EXPECT_EQ(bigint(my_long), 100);
     EXPECT_EQ(bigint(-my_long), -100);
 
@@ -153,6 +162,9 @@ TEST(Test_BigInt, Creation_Tests) {
     EXPECT_EQ(bigint(my_string), "100");
     EXPECT_EQ(bigint(my_string3), "-9223372036854775808");
     EXPECT_EQ(bigint(my_string4), "-9223372036854775809");
+
+    EXPECT_EQ(bigint(hex_string), "269202023463611101042363418");
+    EXPECT_EQ(bigint(neg_hex_string), "-48879");
 }
 
 TEST(Test_BigInt, Unary_Tests) {