pattern
<pre>
  (abc|def){2,4}
</pre>
is compiled as if it were
<pre>
  (abc|def)(abc|def)((abc|def)(abc|def)?)?
</pre>
(Technical aside: It is done this way so that backtrack points within each of
the repetitions can be independently maintained.)
</P>
<P>
For regular expressions whose quantifiers use only small numbers, this is not
usually a problem. However, if the numbers are large, and particularly if such
repetitions are nested, the memory usage can become an embarrassment. For
example, the very simple