diff --git a/string2.lua b/string2.lua
index dd0af98..034c751 100644
--- a/string2.lua
+++ b/string2.lua
@@ -131,7 +131,7 @@ local string2 = {
       end
     end)
     str = gsub(str, "^_*%d", "_%1")
-    if match(str, "^_*[a-z]*$") then
+    if match(str, "^_*[abdefgilnortuw][aefhilnoru][acdefiklnoprstu]*$") then
       for _index_0 = 1, #lua_keywords do
         local kw = lua_keywords[_index_0]
         if match(str, ("^_*" .. kw)) then
@@ -142,7 +142,7 @@ local string2 = {
     return str
   end,
   from_lua_id = function(str)
-    if match(str, "^_+[a-z]*$") then
+    if match(str, "^_+[abdefgilnortuw][aefhilnoru][acdefiklnoprstu]*$") then
       for _index_0 = 1, #lua_keywords do
         local kw = lua_keywords[_index_0]
         if match(str, ("^_+" .. kw)) then
diff --git a/string2.moon b/string2.moon
index 426504c..27e7122 100644
--- a/string2.moon
+++ b/string2.moon
@@ -64,7 +64,8 @@ string2 = {
             else format("x%02X", byte(c))
         -- Lua IDs can't start with numbers, so map "1" -> "_1", "_1" -> "__1", etc.
         str = gsub str, "^_*%d", "_%1"
-        if match str, "^_*[a-z]*$"
+        -- This pattern is guaranteed to match all keywords, but also matches some other stuff.
+        if match str, "^_*[abdefgilnortuw][aefhilnoru][acdefiklnoprstu]*$"
             for kw in *lua_keywords
                 if match str, ("^_*"..kw)
                     str = "_"..str
@@ -73,7 +74,8 @@ string2 = {
     -- from_lua_id(as_lua_id(str)) == str, but behavior is unspecified for inputs that
     -- did not come from as_lua_id()
     from_lua_id: (str)->
-        if match str, "^_+[a-z]*$"
+        -- This pattern is guaranteed to match all keywords, but also matches some other stuff.
+        if match str, "^_+[abdefgilnortuw][aefhilnoru][acdefiklnoprstu]*$"
             for kw in *lua_keywords
                 if match str, ("^_+"..kw)
                     str = str\sub(2,-1)