try the other way around

init.lua

@@ -111,3 +111,7 @@ minetest.register_allow_player_inventory_action(function(player, action, inv, da
 		return 0
 	end
 end)
+
+if true then
+	dofile(skins.modpath.."/unittest.lua")
+end

skinlist.lua

@@ -2,7 +2,8 @@ local dbgprint = false and print or function() end
 
 --- @param path     Path to the "textures" directory, without tailing slash.
 --- @param filename Current file name, such as "player.groot.17.png".
-local function process_skin_texture(path, filename)
+--- @return On error: false, error message. On success: true, skin key
+function skins.register_skin(path, filename)
 	-- See "textures/readme.txt" for allowed formats
 
 	local prefix, sep, identifier, extension = filename:match("^(%a+)([_.])([%w_.]+)%.(%a+)$")
@@ -16,17 +17,21 @@ local function process_skin_texture(path, filename)
 
 	-- Filter out files that do not match the allowed patterns
 	if not extension or extension:lower() ~= "png" then
-		return -- Not a skin texture
+		return false, "invalid skin name"
 	end
 	if prefix ~= "player" and prefix ~= "character" then
-		return -- Unknown type
+		return false, "unknown type"
 	end
 
 	local preview_suffix = sep .. "preview"
 	if identifier:sub(-#preview_suffix) == preview_suffix then
-		-- skip preview textures
-		-- This is added by the main skin texture (if exists)
-		return
+		-- The preview texture is added by the main skin texture (if exists)
+		return false, "preview texture"
+	end
+
+	assert(path)
+	if path == ":UNITTEST:" then
+		path = nil
 	end
 
 	dbgprint("Found skin", prefix, identifier, extension)
@@ -58,12 +63,16 @@ local function process_skin_texture(path, filename)
 	local skin_obj = skins.get(filename_noext) or skins.new(filename_noext)
 	skin_obj:set_texture(filename)
 	skin_obj:set_meta("_sort_id", sort_id)
+	if sep ~= "_" then
+		skin_obj._legacy_name = filename_noext:gsub("[._]+", "_")
+	end
+
 	if playername then
 		skin_obj:set_meta("assignment", "player:"..playername)
 		skin_obj:set_meta("playername", playername)
 	end
 
-	do
+	if path then
 		-- Get type of skin based on dimensions
 		local file = io.open(path .. "/" .. filename, "r")
 		local skin_format = skins.get_skin_format(file)
@@ -74,7 +83,7 @@ local function process_skin_texture(path, filename)
 	skin_obj:set_hand_from_texture()
 	skin_obj:set_meta("name", identifier)
 
-	do
+	if path then
 		-- Optional skin information
 		local file = io.open(path .. "/../meta/" .. filename_noext .. ".txt", "r")
 		if file then
@@ -86,7 +95,7 @@ local function process_skin_texture(path, filename)
 		end
 	end
 
-	do
+	if path then
 		-- Optional preview texture
 		local preview_name = filename_noext .. sep .. "preview.png"
 		local fh = io.open(path .. "/" .. preview_name)
@@ -95,6 +104,8 @@ local function process_skin_texture(path, filename)
 			skin_obj:set_preview(preview_name)
 		end
 	end
+
+	return true, skin_obj:get_key()
 end
 
 --- Internal function. Fallback/migration code for `.`-delimited skin names that
@@ -111,43 +122,12 @@ function skins.__fuzzy_match_skin_name(player_name, skin_name, be_noisy)
 		return
 	end
 
-	--[[
-		Public skin names:
-			character_[number]
-		Private skin names:
-			player_sam_doe
-			player_sam_doe_1234 <-- might also belong to player "sam_doe_1233"
-			.      ^^^^^^^^^^^^ identifier
-			^^^^^^------------- prefix
-
-		Approach: try to find a match for the longest possible name
-		Assumption 1: There are no skin variations without numeric ending
-		Assumption 2: Player names to not end with `_%d+`
-	]]
-
-	local prefix, identifier = unpack(skin_name:split("_", true, 1))
-	local for_player, number
-	if prefix == "player" then
-		for_player, number = identifier:match("^([%w_]+)_(%d+)$")
-		if not number then
-			for_player = identifier
+	for _, skin in pairs(skins.meta) do
+		if skin._legacy_name == skin_name then
+			dbgprint("Match", skin_name, skin:get_key())
+			return skin
 		end
-	else -- character
-		number = identifier
-	end
-
-	local skin = nil
-	if not for_player or for_player == player_name then
-		skin = skins.get(prefix
-			.. (for_player and ("." .. for_player) or "")
-			.. (number and ("." .. number) or "")
-		)
-	end
-
-	if skin then
-		-- MATCH!
-		dbgprint("Match", skin_name, skin:get_key())
-		return skin
+		--dbgprint("Try match", skin_name, skin:get_key(), skin._legacy_name)
 	end
 
 	if be_noisy then
@@ -162,7 +142,7 @@ do
 	local skins_dir_list = minetest.get_dir_list(skins_path)
 
 	for _, fn in pairs(skins_dir_list) do
-		process_skin_texture(skins_path, fn)
+		skins.register_skin(skins_path, fn)
 	end
 end
 

unittest.lua

@@ -0,0 +1,53 @@
+-- Ensure a best possible compatibility with `_` underscore delimiter weirdnesses
+
+local function get_skin(skin_name)
+	local skin = skins.get(skin_name)
+		or skins.__fuzzy_match_skin_name("(unittest)", skin_name, true)
+	return skin and skin:get_key() or nil
+end
+
+local function run_unittest()
+	local PATH = ":UNITTEST:"
+
+	-- -----
+	-- `.`: Simple register + retrieve operations
+	skins.register_skin(PATH, "player.DotSep.png")
+	skins.register_skin(PATH, "player._DotSep_666_.1.png")
+
+	assert(get_skin("player.DotSep"))
+	assert(get_skin("player._DotSep_666_.1"))
+	assert(get_skin("player.DotSep.1") == nil)
+
+	-- -----
+	-- Ambiguous skin names (filenames without extension). Register + retrieve
+	skins.new("player_AmbSki")
+	skins.new("player_AmbSki_1")
+	skins.new("player_AmbSki_666_1")
+
+	assert(get_skin("player_AmbSki"))
+	assert(get_skin("player_AmbSki_") == nil)
+	assert(get_skin("player_AmbSki_1"))
+	assert(get_skin("player_AmbSki_666_1"))
+	-- There are no `__` patterns as they were silently removed by string.split
+
+
+	-- -----
+	-- Mod Storage backwards compatibility
+	-- Match the old `_` notation to `.`-separated skins
+	skins.register_skin(PATH, "player.ComPat42.png")
+	skins.register_skin(PATH, "player.ComPat42.5.png")
+	skins.register_skin(PATH, "player._Com_Pat_42.png")
+	skins.register_skin(PATH, "player._Com_Pat_42.1.png")
+
+	assert(get_skin("player_ComPat42") == "player.ComPat42")
+	assert(get_skin("player_ComPat42_5") == "player.ComPat42.5")
+	assert(get_skin("player_Com_Pat_42") == "player._Com_Pat_42")
+	assert(get_skin("player_Com_Pat_42_1") == "player._Com_Pat_42.1")
+
+
+	error("Unittest passed! Please disable them now.")
+end
+
+
+--run_unittest()
+