diff --git a/src/lexer.c b/src/lexer.c
index e2854f34..7f3c2b24 100644
--- a/src/lexer.c
+++ b/src/lexer.c
@@ -42,8 +42,8 @@ bool is_digit(char c)
 
 bool is_hex(char c)
 {
-    return ((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || c == 'x' ||
-            (c >= 'A' && c <= 'F'));
+    return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') ||
+           (c >= 'A' && c <= 'F');
 }
 
 bool is_numeric(char buffer[])
@@ -51,10 +51,10 @@ bool is_numeric(char buffer[])
     bool hex = false;
     int size = strlen(buffer);
 
-    if (size > 2)
-        hex = !strncmp(buffer, "0x", 2);
+    if (size > 2 && buffer[0] == '0' && (buffer[1] | 32) == 'x')
+        hex = true;
 
-    for (int i = 0; i < size; i++) {
+    for (int i = hex ? 2 : 0; i < size; i++) {
         if (hex && !is_hex(buffer[i]))
             return false;
         if (!hex && !is_digit(buffer[i]))
@@ -177,9 +177,38 @@ token_t lex_token_internal(bool aliasing)
 
     if (is_digit(next_char)) {
         int i = 0;
-        do {
+        token_str[i++] = next_char;
+        read_char(false);
+
+        if (token_str[0] == '0' && ((next_char | 32) == 'x')) {
+            /* Hexadecimal: starts with 0x or 0X */
             token_str[i++] = next_char;
-        } while (is_hex(read_char(false)));
+
+            read_char(false);
+            if (!is_hex(next_char))
+                error("Invalid hex literal: expected hex digit after 0x");
+
+            do {
+                token_str[i++] = next_char;
+            } while (is_hex(read_char(false)));
+
+        } else if (token_str[0] == '0') {
+            /* Octal: starts with 0 but not followed by 'x' */
+            while (is_digit(next_char)) {
+                if (next_char >= '8')
+                    error("Invalid octal digit: must be in range 0-7");
+                token_str[i++] = next_char;
+                read_char(false);
+            }
+
+        } else {
+            /* Decimal */
+            while (is_digit(next_char)) {
+                token_str[i++] = next_char;
+                read_char(false);
+            }
+        }
+
         token_str[i] = 0;
         skip_whitespace();
         return T_numeric;
diff --git a/src/parser.c b/src/parser.c
index afa9b2ed..be032d18 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -295,7 +295,7 @@ int read_numeric_constant(char buffer[])
     int i = 0;
     int value = 0;
     while (buffer[i]) {
-        if (i == 1 && (buffer[i] == 'x')) { /* hexadecimal */
+        if (i == 1 && (buffer[i] | 32) == 'x') { /* hexadecimal */
             value = 0;
             i = 2;
             while (buffer[i]) {
@@ -784,7 +784,7 @@ void read_numeric_param(block_t *parent, basic_block_t *bb, int is_neg)
         i++;
     }
     if (token[0] == '0') {
-        if (token[1] == 'x') { /* hexdecimal */
+        if ((token[1] | 32) == 'x') { /* hexdecimal */
             i = 2;
             do {
                 c = token[i++];
diff --git a/tests/driver.sh b/tests/driver.sh
index b32eee4d..fc1bbfed 100755
--- a/tests/driver.sh
+++ b/tests/driver.sh
@@ -1855,4 +1855,32 @@ int main(void)
 }
 EOF
 
+try_output 0 "2748 6719 105884 0" << EOF
+int main()
+{
+    int a = 0XABC;
+    int b = 0X1a3f;
+    int c = 0XDEaD + 0xBeEF;
+    int d = 0X0;
+    printf("%d %d %d %d", a, b, c, d);
+    return 0;
+}
+EOF
+
+try_compile_error << EOF
+int main()
+{
+    int x = 0X;
+    return 0;
+}
+EOF
+
+try_compile_error << EOF
+int main()
+{
+    int x = 0XGHI;
+    return 0;
+}
+EOF
+
 echo OK