diff --git a/mrbgems/mruby-regexp/mrblib/string_regexp.rb b/mrbgems/mruby-regexp/mrblib/string_regexp.rb
index e15e507a3a..c63e6e3b01 100644
--- a/mrbgems/mruby-regexp/mrblib/string_regexp.rb
+++ b/mrbgems/mruby-regexp/mrblib/string_regexp.rb
@@ -35,21 +35,29 @@ def gsub(pattern, replacement = nil, &block)
     end
     # block case: keep in Ruby to avoid VM callback from C
     parts = []
-    rest = self
-    while rest.length > 0
-      md = pattern.match(rest)
+    pos = 0
+    len = self.bytesize
+    while pos <= len
+      md = pattern.match(self, pos)
       break unless md
-      parts << md.pre_match
+      match_start = md.begin(0)
+      match_end = md.end(0)
+      parts << self.byteslice(pos, match_start - pos)
       parts << block.call(md[0]).to_s
-      matched_len = md[0].length
-      if matched_len == 0
-        parts << rest[0] if rest.length > 0
-        rest = rest[1..-1] || ""
+      if match_start == match_end
+        rest = self.byteslice(match_end..-1)
+        if rest && rest.bytesize > 0
+          char = rest[0]
+          parts << char
+          pos = match_end + char.bytesize
+        else
+          pos = match_end + 1
+        end
       else
-        rest = md.post_match
+        pos = match_end
       end
     end
-    parts << rest
+    parts << self.byteslice(pos..-1)
     parts.join
   end
 
diff --git a/mrbgems/mruby-regexp/src/re_exec.c b/mrbgems/mruby-regexp/src/re_exec.c
index 8f6e047319..39dd39165a 100644
--- a/mrbgems/mruby-regexp/src/re_exec.c
+++ b/mrbgems/mruby-regexp/src/re_exec.c
@@ -145,7 +145,7 @@ add_thread(pike_state *s, re_threadlist *list,
       continue;
 
     case RE_BOL:
-      if (sp == s->str || ((s->pat->flags & RE_FLAG_MULTILINE) && sp > s->str && sp[-1] == '\n')) {
+      if (sp == s->str || ((s->pat->flags & RE_FLAG_MULTILINE) && sp > s->str && sp < s->str_end && sp[-1] == '\n')) {
         pc++; continue;
       }
       return;
@@ -452,7 +452,7 @@ bt_match(const mrb_regexp_pattern *pat, const char *str, const char *str_end,
       }
 
     case RE_BOL:
-      if (sp != str && !(pat->flags & RE_FLAG_MULTILINE && sp > str && sp[-1] == '\n')) return FALSE;
+      if (sp != str && !(pat->flags & RE_FLAG_MULTILINE && sp > str && sp < str_end && sp[-1] == '\n')) return FALSE;
       pc++;
       break;
 
diff --git a/mrbgems/mruby-regexp/test/regexp.rb b/mrbgems/mruby-regexp/test/regexp.rb
index e84ead98d4..5c20700a00 100644
--- a/mrbgems/mruby-regexp/test/regexp.rb
+++ b/mrbgems/mruby-regexp/test/regexp.rb
@@ -349,6 +349,20 @@
   assert_equal "HELLO WORLD", "hello world".gsub(/\w+/) { |m| m.upcase }
 end
 
+assert("String#gsub with block and zero-width match") do
+  assert_equal "!abc", "abc".gsub(/^/) { "!" }
+  assert_equal "a!bc", "abc".gsub(/(?=b)/) { "!" }
+  assert_equal "!a!b!c!", "abc".gsub(//) { "!" }
+  assert_equal "!\n", "\n".gsub(/^/m) { "!" }
+  assert_equal "!a\n", "a\n".gsub(/^/m) { "!" }
+  assert_equal "!a\n!b", "a\nb".gsub(/^/m) { "!" }
+  if __ENCODING__ == "UTF-8"
+    assert_equal "！いろは", "いろは".gsub(/^/) { "！" }
+    assert_equal "い！ろは", "いろは".gsub(/(?=ろ)/) { "！" }
+    assert_equal "！い！ろ！は！", "いろは".gsub(//) { "！" }
+  end
+end
+
 assert("String#gsub date reformat") do
   result = "2026-03-21".gsub(/(\d+)-(\d+)-(\d+)/) { "#{$~[3]}/#{$~[2]}/#{$~[1]}" }
   assert_equal "21/03/2026", result