The Match-zero-or-more Operator (`*')
-------------------------------------

  This operator repeats the smallest possible preceding regular
expression as many times as necessary (including zero) to match the
pattern.  `*' represents this operator.  For example, `o*' matches any
string made up of zero or more `o's.  Since this operator operates on
the smallest preceding regular expression, `fo*' has a repeating `o',
not a repeating `fo'.  So, `fo*' matches `f', `fo', `foo', and so on.

  Since the match-zero-or-more operator is a suffix operator, it may be
useless as such when no regular expression precedes it.  This is the
case when it:

   * is first in a regular expression, or

   * follows a match-beginning-of-line, open-group, or alternation
     operator.

Three different things can happen in these cases:

  1. If the syntax bit `RE_CONTEXT_INVALID_OPS' is set, then the
     regular expression is invalid.

  2. If `RE_CONTEXT_INVALID_OPS' isn't set, but `RE_CONTEXT_INDEP_OPS'
     is, then `*' represents the match-zero-or-more operator (which
     then operates on the empty string).

  3. Otherwise, `*' is ordinary.


  The matcher processes a match-zero-or-more operator by first matching
as many repetitions of the smallest preceding regular expression as it
can.  Then it continues to match the rest of the pattern.

  If it can't match the rest of the pattern, it backtracks (as many
times as necessary), each time discarding one of the matches until it
can either match the entire pattern or be certain that it cannot get a
match.  For example, when matching `ca*ar' against `caaar', the matcher
first matches all three `a's of the string with the `a*' of the regular
expression.  However, it cannot then match the final `ar' of the
regular expression against the final `r' of the string.  So it
backtracks, discarding the match of the last `a' in the string.  It can
then match the remaining `ar'.