What Gets Matched?
******************

  Regex usually matches strings according to the "leftmost longest"
rule; that is, it chooses the longest of the leftmost matches.  This
does not mean that for a regular expression containing subexpressions
that it simply chooses the longest match for each subexpression, left to
right; the overall match must also be the longest possible one.

  For example, `(ac*)(c*d[ac]*)\1' matches `acdacaaa', not `acdac', as
it would if it were to choose the longest match for the first
subexpression.