diff --git a/kong-3.5.1-0.rockspec b/kong-3.5.1-0.rockspec
index 90612fe82a14..60d5f6310296 100644
--- a/kong-3.5.1-0.rockspec
+++ b/kong-3.5.1-0.rockspec
@@ -164,6 +164,7 @@ build = {
     ["kong.tools.protobuf"] = "kong/tools/protobuf.lua",
     ["kong.tools.mime_type"] = "kong/tools/mime_type.lua",
     ["kong.tools.request_aware_table"] = "kong/tools/request_aware_table.lua",
+    ["kong.tools.string"] = "kong/tools/string.lua",
 
     ["kong.runloop.handler"] = "kong/runloop/handler.lua",
     ["kong.runloop.events"] = "kong/runloop/events.lua",
diff --git a/kong/pdk/request.lua b/kong/pdk/request.lua
index a5fc2f04d7d3..06fb846a2ae6 100644
--- a/kong/pdk/request.lua
+++ b/kong/pdk/request.lua
@@ -41,9 +41,6 @@ local get_body_file = req.get_body_file
 local decode_args = ngx.decode_args
 
 
-local is_http_subsystem = ngx and ngx.config.subsystem == "http"
-
-
 local PHASES = phase_checker.phases
 
 
@@ -85,19 +82,7 @@ local function new(self)
     end
   end
 
-  local replace_dashes do
-    -- 1.000.000 iterations with input of "my-header":
-    -- string.gsub:        81ms
-    -- ngx.re.gsub:        74ms
-    -- loop/string.buffer: 28ms
-    -- str_replace_char:   14ms
-    if is_http_subsystem then
-      local str_replace_char = require("resty.core.utils").str_replace_char
-      replace_dashes = function(str)
-        return str_replace_char(str, "-", "_")
-      end
-    end
-  end
+  local replace_dashes = require("kong.tools.string").replace_dashes
 
 
   ---
diff --git a/kong/router/atc.lua b/kong/router/atc.lua
index 653f09af2b58..7c59cba03b4d 100644
--- a/kong/router/atc.lua
+++ b/kong/router/atc.lua
@@ -562,6 +562,7 @@ local get_queries_key
 do
   local tb_sort = table.sort
   local tb_concat = table.concat
+  local replace_dashes_lower = require("kong.tools.string").replace_dashes_lower
 
   local str_buf = buffer.new(64)
 
@@ -570,7 +571,7 @@ do
 
     -- NOTE: DO NOT yield until str_buf:get()
     for name, value in pairs(headers) do
-      local name = name:gsub("-", "_"):lower()
+      local name = replace_dashes_lower(name)
 
       if type(value) == "table" then
         for i, v in ipairs(value) do
diff --git a/kong/router/compat.lua b/kong/router/compat.lua
index dc0b5cdd08e9..6da3522f4698 100644
--- a/kong/router/compat.lua
+++ b/kong/router/compat.lua
@@ -10,7 +10,8 @@ local tb_nkeys = require("table.nkeys")
 local uuid = require("resty.jit-uuid")
 
 
-local shallow_copy    = require("kong.tools.utils").shallow_copy
+local shallow_copy          = require("kong.tools.utils").shallow_copy
+local replace_dashes_lower  = require("kong.tools.string").replace_dashes_lower
 
 
 local is_regex_magic  = utils.is_regex_magic
@@ -251,7 +252,7 @@ local function get_expression(route)
       single_header_buf:reset():put("(")
 
       for i, value in ipairs(v) do
-        local name = "any(http.headers." .. h:gsub("-", "_"):lower() .. ")"
+        local name = "any(http.headers." .. replace_dashes_lower(h) .. ")"
         local op = OP_EQUAL
 
         -- value starts with "~*"
diff --git a/kong/tools/string.lua b/kong/tools/string.lua
new file mode 100644
index 000000000000..3ed03a5d293a
--- /dev/null
+++ b/kong/tools/string.lua
@@ -0,0 +1,45 @@
+local find          = string.find
+local gsub          = string.gsub
+
+
+local _M = {}
+
+
+local replace_dashes
+local replace_dashes_lower
+do
+  local str_replace_char
+
+  if ngx and ngx.config.subsystem == "http" then
+
+    -- 1,000,000 iterations with input of "my-header":
+    -- string.gsub:        81ms
+    -- ngx.re.gsub:        74ms
+    -- loop/string.buffer: 28ms
+    -- str_replace_char:   14ms
+    str_replace_char = require("resty.core.utils").str_replace_char
+
+  else    -- stream subsystem
+    str_replace_char = function(str, ch, replace)
+      if not find(str, ch, nil, true) then
+        return str
+      end
+
+      return gsub(str, ch, replace)
+    end
+  end
+
+  replace_dashes = function(str)
+    return str_replace_char(str, "-", "_")
+  end
+
+  replace_dashes_lower = function(str)
+    return str_replace_char(str:lower(), "-", "_")
+  end
+end
+_M.replace_dashes = replace_dashes
+_M.replace_dashes_lower = replace_dashes_lower
+
+
+return _M
+