diff --git a/include/constructors/constructors.hpp b/include/constructors/constructors.hpp
index 6e6ebbe..dff2114 100644
--- a/include/constructors/constructors.hpp
+++ b/include/constructors/constructors.hpp
@@ -9,6 +9,7 @@
 
 #include "BigInt.hpp"
 #include "functions/utility.hpp"
+#include "operators/arithmetic_assignment.hpp"
 
 
 /*
@@ -47,31 +48,81 @@ BigInt::BigInt(const long long& num) {
 }
 
 
+/*
+    Anonymous namespace
+    -----------------
+    Contains helper functions for constructing an BigInt from Strings
+*/
+
+namespace {
+
+/*
+    parse_hex_string
+    --------------------
+    Parses the provided string store the result in magnitude.
+    str is assumed to have no sign. str must be in hex. First two chars are ignored
+*/
+
+    void parse_hex_string(std::string &&str, std::string &magnitude) {
+        BigInt res;
+        for (size_t idx = 2; idx < str.size(); idx++) {
+            res *= 16;
+            char c = str[idx];
+            // digit
+            if (c >= '0' && c <= '9')
+                res += c - '0';
+                // lowercase hex
+            else if (c >= 'a' && c <= 'f')
+                res += c - 'a' + 10;
+                // uppercase hex
+            else if (c >= 'A' && c <= 'F')
+                res += c - 'A' + 10;
+            else
+                throw std::invalid_argument("Expected a hex integer, got \'" + str + "\'");
+        }
+        magnitude = res.to_string();
+    }
+
+
+/*
+    parse_string
+    --------------------
+    Parses the provided string store the result in magnitude.
+    str is assumed to have no sign. str can be given in hex by prepending "0x"
+*/
+
+    void parse_string(std::string &&str, std::string &magnitude) {
+        if (str.size() > 1 && str[0] == '0' && str[1] == 'x') {
+            parse_hex_string(std::move(str), magnitude);
+            return;
+        }
+
+        // check if every char is decimal
+        for (auto c : str)
+            if (c < '0' || c > '9')
+                throw std::invalid_argument("Expected a decimal integer, got \'" + str + "\'");
+
+        magnitude = std::move(str);
+    }
+
+} // namespace
+
 /*
     String to BigInt
     ----------------
 */
 
 BigInt::BigInt(const std::string& num) {
-    if (num[0] == '+' or num[0] == '-') {     // check for sign
-        std::string magnitude = num.substr(1);
-        if (is_valid_number(magnitude)) {
-            value = magnitude;
+    std::string magnitude = num;
+    sign = '+';
+    if (magnitude[0] == '+' or magnitude[0] == '-') {
             sign = num[0];
-        }
-        else {
-            throw std::invalid_argument("Expected an integer, got \'" + num + "\'");
-        }
-    }
-    else {      // if no sign is specified
-        if (is_valid_number(num)) {
-            value = num;
-            sign = '+';    // positive by default
-        }
-        else {
-            throw std::invalid_argument("Expected an integer, got \'" + num + "\'");
-        }
+            // delete first char
+            magnitude.erase(0,1);
     }
+
+    parse_string(std::move(magnitude), value);
+
     strip_leading_zeroes(value);
 }
 
diff --git a/include/functions/utility.hpp b/include/functions/utility.hpp
index a26ed45..83ec121 100644
--- a/include/functions/utility.hpp
+++ b/include/functions/utility.hpp
@@ -10,21 +10,6 @@
 #include <tuple>
 
 
-/*
-    is_valid_number
-    ---------------
-    Checks whether the given string is a valid integer.
-*/
-
-bool is_valid_number(const std::string& num) {
-    for (char digit : num)
-        if (digit < '0' or digit > '9')
-            return false;
-
-    return true;
-}
-
-
 /*
     strip_leading_zeroes
     --------------------
diff --git a/test/constructors/constructors.cpp b/test/constructors/constructors.cpp
index 7bfd773..1d9bb64 100644
--- a/test/constructors/constructors.cpp
+++ b/test/constructors/constructors.cpp
@@ -22,6 +22,25 @@ TEST_CASE("Construct zero-valued BigInts", "[constructors]") {
     REQUIRE(num4 == 0);
 }
 
+TEST_CASE("Construct BigInts from hex strings", "[constructors]") {
+    BigInt num1_hex("0x0");        // should be 0 by default
+    BigInt num1_dec("0");
+    REQUIRE(num1_hex == 0);
+    REQUIRE(num1_hex == num1_dec);
+
+    BigInt num2("0xfa54");     // 0xf54 passed as an integer
+    REQUIRE(num2 == 0xfa54);
+
+    BigInt num3("0x0f7ab4d822924430b1b97859af0eaa41");
+    REQUIRE(num3 == "20575548111226062968771475379166554689");
+
+    BigInt num4("-0x0f7ab4d822924430b1b97859af0eaa41");
+    REQUIRE(num4 == "-20575548111226062968771475379166554689");
+
+    BigInt num5("-0x0F7AB4D822924430B1B97859AF0EAA41");
+    REQUIRE(num5 == "-20575548111226062968771475379166554689");
+}
+
 TEST_CASE("Randomly construct BigInts from integers and strings",
           "[constructors][random][integer][string]") {
     std::random_device generator;
@@ -39,13 +58,21 @@ TEST_CASE("Randomly construct BigInts from integers and strings",
         BigInt trouble("123BigInt");    // without sign
     }
     catch (std::logic_error &e) {
-        CHECK(e.what() == std::string("Expected an integer, got \'123BigInt\'"));
+        CHECK(e.what() == std::string("Expected a decimal integer, got \'123BigInt\'"));
     }
     try {
         BigInt trouble("-4a5b6c");      // with sign
                                         // looks like hex, but is still not allowed
     }
     catch (std::logic_error &e) {
-        CHECK(e.what() == std::string("Expected an integer, got \'-4a5b6c\'"));
+        CHECK(e.what() == std::string("Expected a decimal integer, got \'4a5b6c\'"));
+    }
+
+    try {
+        BigInt trouble("0x4a5b6cg7923");   // with sign
+                                            // looks like hex, but contains illegal g
+    }
+    catch (std::logic_error &e) {
+        CHECK(e.what() == std::string("Expected a hex integer, got \'0x4a5b6cg7923\'"));
     }
 }