diff --git a/lua/pac3/libraries/expression.lua b/lua/pac3/libraries/expression.lua
index f44ad9655..216fd137d 100644
--- a/lua/pac3/libraries/expression.lua
+++ b/lua/pac3/libraries/expression.lua
@@ -1,4 +1,3 @@
-
 local lib = {
 	PI = math.pi,
 	rand = math.random,
@@ -37,34 +36,108 @@ local lib = {
 	round = math.Round,
 }
 
-local blacklist = {"repeat", "until", "function", "end"}
+local blacklist = {
+    "function";
+    "for"; "break";
+    "while"; "do";
+    "repeat"; "until";
+}
 
-local function compile_expression(str, extra_lib)
-	for _, word in pairs(blacklist) do
-		if str:find("[%p%s]" .. word) or str:find(word .. "[%p%s]") then
-			return false, string.format("illegal characters used %q", word)
+-- convert blacklist items into patterns to match
+for k, item in pairs(blacklist) do
+    -- uses more efficient / conclusive frontier pattern syntax
+    -- frontier matches transition into and out of sets
+    -- in this case, matching transition into letters and then out of letters (to match whole words and not partials)
+    blacklist[k] = ("%%f[%%a](%s)%%f[%%A]"):format(item)
+end
+
+local function_intro = "local IN = (...); "
+
+local function_formats = {
+    function_intro .. "return %s";
+    function_intro .. "%s"; -- allows <code>return</code> semantics
+}
+
+local function TryCompile(code, identifier)
+    local result = CompileString(code, identifier, false)
+    
+    return not isstring(result), result
+end
+
+local function CompileStringAdvanced(code, identifier)
+    local success, func = false, nil
+    
+    for _, structure in pairs(function_formats) do
+        success, func = TryCompile(structure:format(code), identifier)
+        
+        if success then break end
+    end
+    
+    return success, func
+end
+
+local function readonlyError()
+    error("Not allowed to assign to globals", 2)
+end
+
+local function makeReadonly(t)
+    return setmetatable(
+        {}, 
+        {
+            __index = t,
+            __newindex = readonlyError,
+            __metatable = "This metatable is locked."
+        }
+    )
+end
+
+local function copyInto(t1, t2)
+    for k,v in pairs(t1) do t2[k] = v end
+end
+
+local function checkBlacklist(code)
+    local str
+    
+    for _, word in pairs(blacklist) do
+        str = code:match(word)
+        
+		if str then
+			return str
 		end
 	end
+    
+    return nil
+end
 
-	local functions = {}
+local function compile_expression(str, extra_lib)
+    local illegalWord = checkBlacklist(str)
+    
+	if illegalWord then
+        return false, string.format("illegal characters used %q", illegalWord)
+    end
 
-	for k,v in pairs(lib) do functions[k] = v end
+	local success, func = CompileStringAdvanced(str, "pac_expression")
 
-	if extra_lib then
-		for k,v in pairs(extra_lib) do functions[k] = v end
-	end
+	if success then
+        local functions = {}
 
-	functions.select = select
-	str = "local IN = select(1, ...) return " .. str
+        copyInto(lib, functions)
 
-	local func = CompileString(str, "pac_expression", false)
+        if extra_lib then
+            copyInto(extra_lib, functions)
+        end
 
-	if isstring(func) then
-		return false, func
-	else
-		setfenv(func, functions)
+        functions.select = select
+        
+        setfenv(
+            func, 
+            makeReadonly(functions)
+        )
+        
 		return true, func
 	end
+    
+    return false, func
 end
 
 return compile_expression