Rrerku

I'd like to draw the Chomsky hierarchy using tikz, similar to:

I have the following so far, for a subset of the hierarchy:

documentclass[11pt]article
usepackagetikz
usetikzlibraryshapes,arrows,calc
% ----------

begindocument
 begintikzpicture
 tikzstylevenn = [ellipse, minimum height=3em, minimum width=12em, draw]
 node [venn] (cs) at (0, $re.south$) Context Sensitive;
 node [venn, minimum height=10em] (re) at (0,0) Recursively Enumerable;
 endtikzpicture
enddocument

How can I:

1. Anchor all of the shapes to the same point in the centre at the bottom of the diagram?




2. Align the text to the top of each ellipse - is shift appropriate here?

This is not a perfect solution, but it fits this particular case: write words above another node.

documentclass[tikz]standalone
usetikzlibraryshapes
begindocument
begintikzpicture
node[above,ellipse,minimum height=2em,minimum width=4em,draw] (a) regular;
node[above,ellipse,minimum height=4em,minimum width=8em,draw] (b) ;
node[above,ellipse,minimum height=6em,minimum width=12em,draw] (c) ;
node[above,ellipse,minimum height=8em,minimum width=16em,draw] (d) ;
path (a.north) node[above] context-free
 (b.north) node[above] context-sensitive
 (c.north) node[above] recursively enumerable;
endtikzpicture
enddocument

Naively one may think this should be simple with fit but unfortunately ellipse fits are not tight by default. (See here for a tighter fit, but my naive attempt to use it did not yield the desired results.) So one may do it differently. Note that this is not absolutely fool-proof but can be made so with more efforts. However, for the case at hand it works.

documentclass[11pt]article
usepackagetikz
usetikzlibraryshapes.geometric,calc
begindocument
 begintikzpicture[font=sffamily]
 foreach X [count=Y,remember=Y as LastY] in 
 regular,context free,context sensitive,recursively enumerable
 ifnumY=1
 node[ellipse,draw,outer sep=0pt] (F-Y) X;
 else
 node[anchor=south] (T-Y) at (F-LastY.north) X;
 path let p1=($([yshift=1ex]T-Y.north)-(F-LastY.south)$),
 p2=($(F-1.east)-(F-1.west)$),p3=($(F-1.north)-(F-1.south)$)
 in ($([yshift=1ex]T-Y.north)!0.5!(F-LastY.south)$) 
 node[minimum height=y1,minimum width=y1*x2/y3,
 draw,ellipse,inner sep=0pt] (F-Y);
 fi
 endtikzpicture
enddocument