Unexpected lua pattern matching result

For the following code:

local function getParentPath(_path)

    pattern = "(.+)([/\\][/\\])(.+)"
    i,j,k = string.match(path,pattern)
    return i,j,k

end

print(getParentPath(path))

For path = "C://data//file.text", I get:

C://data // file.text

But For path = "C:\data\file.text", I get:

nil nil nil

I am looking for a pattern which woks for both. Any suggestions?