Add a "fork" of musl as //fusl.

fusl/src/regex/regerror.c

+#include <string.h>
+#include <regex.h>
+#include <stdio.h>
+#include "locale_impl.h"
+
+/* Error message strings for error codes listed in `regex.h'.  This list
+   needs to be in sync with the codes listed there, naturally. */
+
+/* Converted to single string by Rich Felker to remove the need for
+ * data relocations at runtime, 27 Feb 2006. */
+
+static const char messages[] = {
+  "No error\0"
+  "No match\0"
+  "Invalid regexp\0"
+  "Unknown collating element\0"
+  "Unknown character class name\0"
+  "Trailing backslash\0"
+  "Invalid back reference\0"
+  "Missing ']'\0"
+  "Missing ')'\0"
+  "Missing '}'\0"
+  "Invalid contents of {}\0"
+  "Invalid character range\0"
+  "Out of memory\0"
+  "Repetition not preceded by valid expression\0"
+  "\0Unknown error"
+};
+
+size_t regerror(int e, const regex_t *restrict preg, char *restrict buf, size_t size)
+{
+        const char *s;
+        for (s=messages; e && *s; e--, s+=strlen(s)+1);
+        if (!*s) s++;
+        s = LCTRANS_CUR(s);
+        return 1+snprintf(buf, size, "%s", s);
+}