diff --git a/c-programming/strings/string_case.c b/c-programming/strings/string_case.c
new file mode 100644
index 0000000..de5b756
--- /dev/null
+++ b/c-programming/strings/string_case.c
@@ -0,0 +1,38 @@
+/*
+ * string_case.c
+ *
+ * Author: Intel A80486DX2-66
+ * License: Unlicense
+ */
+
+#include "string_case.h"
+
+char* ascii_lowercase_str(const char* s) { ASCII_LOWERCASE_STR_T(s); }
+char* ascii_lowercase_strm(char* s) { ASCII_LOWERCASE_STRM_T(s); }
+char* ascii_uppercase_str(const char* s) { ASCII_UPPERCASE_STR_T(s); }
+char* ascii_uppercase_strm(char* s) { ASCII_UPPERCASE_STRM_T(s); }
+
+#ifdef TEST
+# include <stdio.h>
+# include <stdlib.h>
+
+int main(void) {
+	printf("Constant string:\n");
+	const char* const_str = "Hello, World!";
+	char* str1 = ascii_lowercase_str(const_str),
+	    * str2 = ascii_uppercase_str(const_str);
+	printf("original string: %s\n", const_str);
+	printf("lower: %s\n", str1); free(str1);
+	printf("upper: %s\n", str2); free(str2);
+	printf("original string: %s\n", const_str);
+
+	printf("\nNon-constant string:\n");
+	char non_const_str[] = "Hello, user!";
+	printf("original string: %s\n", non_const_str);
+	printf("lower: %s\n", ascii_lowercase_strm(non_const_str));
+	printf("upper: %s\n", ascii_uppercase_strm(non_const_str));
+	printf("original string: %s\n", non_const_str);
+
+	return EXIT_SUCCESS;
+}
+#endif
diff --git a/c-programming/strings/string_case.h b/c-programming/strings/string_case.h
new file mode 100644
index 0000000..be6ba74
--- /dev/null
+++ b/c-programming/strings/string_case.h
@@ -0,0 +1,51 @@
+/*
+ * string_case.h
+ *
+ * Author: Intel A80486DX2-66
+ * License: Unlicense
+ */
+
+#ifndef _STRING_CASE_H
+#define _STRING_CASE_H
+
+#ifdef __STDC_ALLOC_LIB__
+# define __STDC_WANT_LIB_EXT2__ 1
+#else
+# define _POSIX_C_SOURCE 200809L
+#endif
+#include <stdlib.h>
+#include <string.h>
+
+#define ASCII_IS_LOWER(c) ((c) >= 'a' && (c) <= 'z')
+#define ASCII_IS_UPPER(c) ((c) >= 'A' && (c) <= 'Z')
+#define ASCII_IS_ALPHA(c) (ASCII_IS_LOWER(c) || ASCII_IS_UPPER(c))
+#define ASCII_MOD_ALPHA(c, v) (ASCII_IS_ALPHA(c) ? v : c)
+#define ASCII_LOWERCASE(c) (ASCII_MOD_ALPHA((c), (c) | 0x20))
+#define ASCII_UPPERCASE(c) (ASCII_MOD_ALPHA((c), ASCII_LOWERCASE(c) - 0x20))
+#define ASCII_MOD_STRM(s, m) \
+  char* ptr = s;                                                              \
+  while (*ptr) {                                                              \
+    *ptr = m(*ptr);                                                           \
+    ptr++;                                                                    \
+  }                                                                           \
+  return s
+
+#define ASCII_MOD_STR(s, m) \
+  char* s_dup = strdup((s)), * ptr = s_dup;                                   \
+  while (*ptr) {                                                              \
+    *ptr = m(*ptr);                                                           \
+    ptr++;                                                                    \
+  }                                                                           \
+  return s_dup
+
+#define ASCII_UPPERCASE_STRM_T(s) ASCII_MOD_STRM(s, ASCII_UPPERCASE)
+#define ASCII_LOWERCASE_STRM_T(s) ASCII_MOD_STRM(s, ASCII_LOWERCASE)
+#define ASCII_UPPERCASE_STR_T(s) ASCII_MOD_STR(s, ASCII_UPPERCASE)
+#define ASCII_LOWERCASE_STR_T(s) ASCII_MOD_STR(s, ASCII_LOWERCASE)
+
+char* ascii_lowercase_str(const char* s);
+char* ascii_lowercase_strm(char* s);
+char* ascii_uppercase_str(const char* s);
+char* ascii_uppercase_strm(char* s);
+
+#endif /* _STRING_CASE_H */