[Garkin]

Some more suggestions:

local MAJOR, MINOR = "LibConstantMapper-1.0", 1
 
local lib, oldminor = LibStub:NewLibrary(MAJOR, MINOR)
if not lib then return end
 
local mappingsTable = {
   ["ACTION_TYPE"] = { exclude = "DEPRECATED" },
   ["ATTRIBUTE"] ={ exclude = "ATTRIBUTE_.+_" },
   ["CHAT_CATEGORY"] = { exclude = "^CHAT_CATEGORY_HEADER" },
   ["CURRENCY_CHANGE_REASON"] = {},
   ["DAMAGE_TYPE" = {},
   ["ITEM_QUALITY"] = {},
   ["ITEM_TRAIT_TYPE"] = {},
   ["ITEMSTYLE"] = { exclude = "DEPRECATED" },
   ["ITEMTYPE"] = {},
   ["LINK_TYPE"] = { pattern = "_LINK_TYPE$" }
   ["STAT"] = { exclude = {"^STAT_BONUS_OPTION_", "^STAT_SOFT_CAP_OPTION", "^STAT_VALUE_COLOR"} }
}
 
--do not overwrite existing data
lib.data = lib.data or {}
 
--internal functions
function lib:AddData(mapping, pattern, exclude)
   for name, value in zo_insecurePairs(_G) do
      if type(mapping) == "table" then
         for mappingName, mappingTable in pairs(mapping) do
            lib:SaveDataEntry(name, value, mappingName, mappingTable.pattern, mappingTable.exclude)
         end
      elseif type(mapping) == "string" then
         lib:SaveDataEntry(name, value, mapping, pattern, exclude)
      end
   end
end
 
function lib:SaveDataEntry(name, value, mapping, pattern, exclude)
   if (name):find(pattern or "^"..mapping.."_") == nil then return end
 
   local skip = false
   if exclude then 
      if type(exclude) == "table" then
         for i,v in ipairs(exclude) do 
            if (name):find(v) then
               skip = true
               break
            end
         end
      else
         skip = (name):find(exclude) ~= nil
      end
   end
   
   if not skip then 
      lib.data[mapping] = data[mapping] or {}
      lib.data[mapping][name] = value
   end
end
 
if oldminor then
   --library update, add only new/changed items?
   local newItems = {}
   for name, value in pairs(mappingsTable) do
      if value ~= lib.mappings[name] then
         newItems[name] = value
         lib.mappings = value
      end
   end
   lib:AddData(newItems) 
else
   lib.mappings = mappingsTable
   lib:AddData(mappingsTable)
end
 
 
--external functions
function lib:AddMapping(mapping, pattern, exclude)
   --add or replace enxisting entry
   if type(mapping) == "table" then
      for name, value in pairs(mapping) do
         lib.mappings[name] = value
      end
   else  
      lib.mappings[mapping] = { ["pattern"] = pattern, ["exclude"] = exclude }
   end
 
   lib:AddData(mapping, pattern, exclude)   
end
 
function lib:GetConstants(mapping)
   return lib.data[mapping]
end

1) Mapping can be part of the name and pattern is defined only in special cases
2) Do not create a new global table ConstantMapperMappings, just make it accessible using the library reference
3) As merlight said, it would be nice to have direct [value] => "key" mapping, so I changed it that way