diff --git a/include/mruby/internal.h b/include/mruby/internal.h
index 3ec83fa3a..89cd5d52b 100644
--- a/include/mruby/internal.h
+++ b/include/mruby/internal.h
@@ -166,6 +166,7 @@ mrb_value mrb_str_aref(mrb_state *mrb, mrb_value str, mrb_value idx, mrb_value l
 uint32_t mrb_byte_hash(const uint8_t*, mrb_int);
 uint32_t mrb_byte_hash_step(const uint8_t*, mrb_int, uint32_t);
 
+mrb_int mrb_utf8_to_buf(char *buf, uint32_t cp);
 #ifdef MRB_UTF8_STRING
 mrb_int mrb_utf8len(const char *str, const char *end);
 mrb_int mrb_utf8_strlen(const char *str, mrb_int byte_len);
diff --git a/mrbgems/mruby-compiler/core/parse.y b/mrbgems/mruby-compiler/core/parse.y
index 383b23b54..7e5a2a0b8 100644
--- a/mrbgems/mruby-compiler/core/parse.y
+++ b/mrbgems/mruby-compiler/core/parse.y
@@ -5375,30 +5375,8 @@ tokadd(parser_state *p, int32_t c)
     len = 1;
   }
   else {
-    /* Unicode character */
-    c = -c;
-    if (c < 0x80) {
-      utf8[0] = (char)c;
-      len = 1;
-    }
-    else if (c < 0x800) {
-      utf8[0] = (char)(0xC0 | (c >> 6));
-      utf8[1] = (char)(0x80 | (c & 0x3F));
-      len = 2;
-    }
-    else if (c < 0x10000) {
-      utf8[0] = (char)(0xE0 |  (c >> 12)        );
-      utf8[1] = (char)(0x80 | ((c >>  6) & 0x3F));
-      utf8[2] = (char)(0x80 | ( c        & 0x3F));
-      len = 3;
-    }
-    else {
-      utf8[0] = (char)(0xF0 |  (c >> 18)        );
-      utf8[1] = (char)(0x80 | ((c >> 12) & 0x3F));
-      utf8[2] = (char)(0x80 | ((c >>  6) & 0x3F));
-      utf8[3] = (char)(0x80 | ( c        & 0x3F));
-      len = 4;
-    }
+    /* Unicode character (negative c indicates codepoint) */
+    len = (int)mrb_utf8_to_buf(utf8, (uint32_t)(-c));
   }
   if (p->tidx+len >= p->tsiz) {
     if (p->tsiz >= MRB_PARSER_TOKBUF_MAX) {
diff --git a/mrbgems/mruby-compiler/core/y.tab.c b/mrbgems/mruby-compiler/core/y.tab.c
index 4633c330b..e27edf8ee 100644
--- a/mrbgems/mruby-compiler/core/y.tab.c
+++ b/mrbgems/mruby-compiler/core/y.tab.c
@@ -12613,30 +12613,8 @@ tokadd(parser_state *p, int32_t c)
     len = 1;
   }
   else {
-    /* Unicode character */
-    c = -c;
-    if (c < 0x80) {
-      utf8[0] = (char)c;
-      len = 1;
-    }
-    else if (c < 0x800) {
-      utf8[0] = (char)(0xC0 | (c >> 6));
-      utf8[1] = (char)(0x80 | (c & 0x3F));
-      len = 2;
-    }
-    else if (c < 0x10000) {
-      utf8[0] = (char)(0xE0 |  (c >> 12)        );
-      utf8[1] = (char)(0x80 | ((c >>  6) & 0x3F));
-      utf8[2] = (char)(0x80 | ( c        & 0x3F));
-      len = 3;
-    }
-    else {
-      utf8[0] = (char)(0xF0 |  (c >> 18)        );
-      utf8[1] = (char)(0x80 | ((c >> 12) & 0x3F));
-      utf8[2] = (char)(0x80 | ((c >>  6) & 0x3F));
-      utf8[3] = (char)(0x80 | ( c        & 0x3F));
-      len = 4;
-    }
+    /* Unicode character (negative c indicates codepoint) */
+    len = (int)mrb_utf8_to_buf(utf8, (uint32_t)(-c));
   }
   if (p->tidx+len >= p->tsiz) {
     if (p->tsiz >= MRB_PARSER_TOKBUF_MAX) {
diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index 46b35822a..ee9c8673e 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -1124,24 +1124,7 @@ io_putc(mrb_state *mrb, mrb_value io)
   if (len == 0) return c;
 
 #ifdef MRB_UTF8_STRING
-  /* Determine UTF-8 character length from first byte */
-  unsigned char first = (unsigned char)ptr[0];
-  if (first < 0x80) {
-    write_len = 1;        /* ASCII */
-  }
-  else if ((first & 0xE0) == 0xC0) {
-    write_len = 2;        /* 2-byte UTF-8 */
-  }
-  else if ((first & 0xF0) == 0xE0) {
-    write_len = 3;        /* 3-byte UTF-8 */
-  }
-  else if ((first & 0xF8) == 0xF0) {
-    write_len = 4;        /* 4-byte UTF-8 */
-  }
-  else {
-    write_len = 1;        /* Invalid UTF-8, write single byte */
-  }
-  if (write_len > len) write_len = len;
+  write_len = mrb_utf8len(ptr, ptr + len);
 #else
   write_len = 1;          /* Non-UTF8: write single byte */
 #endif
diff --git a/mrbgems/mruby-pack/src/pack.c b/mrbgems/mruby-pack/src/pack.c
index 4342c87d4..7688927a5 100644
--- a/mrbgems/mruby-pack/src/pack.c
+++ b/mrbgems/mruby-pack/src/pack.c
@@ -10,6 +10,7 @@
 #include <mruby/string.h>
 #include <mruby/variable.h>
 #include <mruby/endian.h>
+#include <mruby/internal.h>
 #include <mruby/presym.h>
 
 #include <ctype.h>
@@ -772,36 +773,11 @@ static int
 pack_utf8(mrb_state *mrb, mrb_value o, mrb_value str, mrb_int sidx, int count, unsigned int flags)
 {
   char utf8[4];
-  int len = 0;
-  uint32_t c = 0;
+  int len;
+  uint32_t c = (uint32_t)mrb_integer(o);
 
-  c = (uint32_t)mrb_integer(o);
-
-  /* Unicode character */
-  /* from mruby-compiler gem */
-  if (c < 0x80) {
-    utf8[0] = (char)c;
-    len = 1;
-  }
-  else if (c < 0x800) {
-    utf8[0] = (char)(0xC0 | (c >> 6));
-    utf8[1] = (char)(0x80 | (c & 0x3F));
-    len = 2;
-  }
-  else if (c < 0x10000) {
-    utf8[0] = (char)(0xE0 |  (c >> 12)        );
-    utf8[1] = (char)(0x80 | ((c >>  6) & 0x3F));
-    utf8[2] = (char)(0x80 | ( c        & 0x3F));
-    len = 3;
-  }
-  else if (c < 0x200000) {
-    utf8[0] = (char)(0xF0 |  (c >> 18)        );
-    utf8[1] = (char)(0x80 | ((c >> 12) & 0x3F));
-    utf8[2] = (char)(0x80 | ((c >>  6) & 0x3F));
-    utf8[3] = (char)(0x80 | ( c        & 0x3F));
-    len = 4;
-  }
-  else {
+  len = (int)mrb_utf8_to_buf(utf8, c);
+  if (len == 0) {
     mrb_raise(mrb, E_RANGE_ERROR, "pack(U): value out of range");
   }
 
diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index 5413f16ad..14dba5606 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -520,28 +520,8 @@ retry:
             /* Integer: encode directly to stack buffer (no allocation) */
             mrb_int code = mrb_integer(val);
 #ifdef MRB_UTF8_STRING
-            if (code < 0x80) {
-              cbuf[0] = (char)code;
-              clen = 1;
-            }
-            else if (code < 0x800) {
-              cbuf[0] = (char)(0xC0 | (code >> 6));
-              cbuf[1] = (char)(0x80 | (code & 0x3F));
-              clen = 2;
-            }
-            else if (code < 0x10000) {
-              cbuf[0] = (char)(0xE0 | (code >> 12));
-              cbuf[1] = (char)(0x80 | ((code >> 6) & 0x3F));
-              cbuf[2] = (char)(0x80 | (code & 0x3F));
-              clen = 3;
-            }
-            else {
-              cbuf[0] = (char)(0xF0 | (code >> 18));
-              cbuf[1] = (char)(0x80 | ((code >> 12) & 0x3F));
-              cbuf[2] = (char)(0x80 | ((code >> 6) & 0x3F));
-              cbuf[3] = (char)(0x80 | (code & 0x3F));
-              clen = 4;
-            }
+            clen = (int)mrb_utf8_to_buf(cbuf, (uint32_t)code);
+            if (clen == 0) clen = 1;  /* invalid codepoint: write single byte */
 #else
             cbuf[0] = (char)(code & 0xff);
             clen = 1;
diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 4e9c95d21..74dc1c7d1 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -51,36 +51,15 @@ int_chr_utf8(mrb_state *mrb, mrb_value num)
   char utf8[4];
   mrb_int len;
   mrb_value str;
-  uint32_t sb_flag = 0;
 
   if (cp < 0 || 0x10FFFF < cp) {
     mrb_raisef(mrb, E_RANGE_ERROR, "%v out of char range", num);
   }
-  if (cp < 0x80) {
-    utf8[0] = (char)cp;
-    len = 1;
-    sb_flag = MRB_STR_SINGLE_BYTE;
-  }
-  else if (cp < 0x800) {
-    utf8[0] = (char)(0xC0 | (cp >> 6));
-    utf8[1] = (char)(0x80 | (cp & 0x3F));
-    len = 2;
-  }
-  else if (cp < 0x10000) {
-    utf8[0] = (char)(0xE0 |  (cp >> 12));
-    utf8[1] = (char)(0x80 | ((cp >>  6) & 0x3F));
-    utf8[2] = (char)(0x80 | ( cp        & 0x3F));
-    len = 3;
-  }
-  else {
-    utf8[0] = (char)(0xF0 |  (cp >> 18));
-    utf8[1] = (char)(0x80 | ((cp >> 12) & 0x3F));
-    utf8[2] = (char)(0x80 | ((cp >>  6) & 0x3F));
-    utf8[3] = (char)(0x80 | ( cp        & 0x3F));
-    len = 4;
-  }
+  len = mrb_utf8_to_buf(utf8, (uint32_t)cp);
   str = mrb_str_new(mrb, utf8, len);
-  mrb_str_ptr(str)->flags |= sb_flag;
+  if (len == 1) {
+    RSTR_SET_ASCII_FLAG(mrb_str_ptr(str));
+  }
   return str;
 }
 #endif
diff --git a/src/string.c b/src/string.c
index 768551d2d..cf758e66a 100644
--- a/src/string.c
+++ b/src/string.c
@@ -316,6 +316,39 @@ mrb_gc_free_str(mrb_state *mrb, struct RString *str)
 #define MASK01 0x01010101ul
 #endif
 
+/*
+ * Encode a Unicode codepoint to UTF-8 bytes.
+ * buf must have at least 4 bytes of space.
+ * Returns the number of bytes written (1-4), or 0 for invalid codepoint.
+ */
+mrb_int
+mrb_utf8_to_buf(char *buf, uint32_t cp)
+{
+  if (cp < 0x80) {
+    buf[0] = (char)cp;
+    return 1;
+  }
+  else if (cp < 0x800) {
+    buf[0] = (char)(0xC0 | (cp >> 6));
+    buf[1] = (char)(0x80 | (cp & 0x3F));
+    return 2;
+  }
+  else if (cp < 0x10000) {
+    buf[0] = (char)(0xE0 | (cp >> 12));
+    buf[1] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    buf[2] = (char)(0x80 | (cp & 0x3F));
+    return 3;
+  }
+  else if (cp <= 0x10FFFF) {
+    buf[0] = (char)(0xF0 | (cp >> 18));
+    buf[1] = (char)(0x80 | ((cp >> 12) & 0x3F));
+    buf[2] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    buf[3] = (char)(0x80 | (cp & 0x3F));
+    return 4;
+  }
+  return 0;  /* invalid codepoint */
+}
+
 #ifdef MRB_UTF8_STRING
 
 #define NOASCII(c) ((c) & 0x80)