Repeated patterns matching zero-length substring

       WARNING: Difficult material (and prose) ahead.  This
       section needs a rewrite.

       Regular expressions provide a terse and powerful
       programming language.  As with most other power tools,
       power comes together with the ability to wreak havoc.

       A common abuse of this power stems from the ability to
       make infinite loops using regular expressions, with
       something as innocuous as:

           'foo' =~ m{ ( o? )* }x;

       The o? can match at the beginning of 'foo', and since the
       position in the string is not moved by the match, o? would
       match again and again due to the * modifier.  Another
       common way to create a similar cycle is with the looping
       modifier //g:

           @matches = ( 'foo' =~ m{ o? }xg );

       or

           print "match: <$&>\n" while 'foo' =~ m{ o? }xg;

       or the loop implied by split().

       However, long experience has shown that many programming
       tasks may be significantly simplified by using repeated
       subexpressions which may match zero-length substrings,
       with a simple example being:

           @chars = split //, $string;           # // is not magic in split
           ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /

       Thus Perl allows the /()/ construct, which forcefully
       breaks the infinite loop.  The rules for this are
       different for lower-level loops given by the greedy
       modifiers *+{}, and for higher-level ones like the /g
       modifier or split() operator.

       The lower-level loops are interrupted when it is detected
       that a repeated expression did match a zero-length
       substring, thus

          m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

       is made equivalent to

          m{   (?: NON_ZERO_LENGTH )*
             |
               (?: ZERO_LENGTH )?
           }x;

       The higher level-loops preserve an additional state
       between iterations: whether the last match was zero-
       length.  To break the loop, the following match after a
       zero-length match is prohibited to have a length of zero.
       This prohibition interacts with backtracking (see the
       section on Backtracking), and so the second best match is
       chosen if the best match is of zero length.

       Say,

           $_ = 'bar';
           s/\w??/<$&>/g;

       results in "<<b><><a><><r><>">.  At each position of the
       string the best match given by non-greedy ?? is the zero-

       length match, and the second best match is what is matched
       by \w.  Thus zero-length matches alternate with one-
       character-long matches.

       Similarly, for repeated m/()/g the second-best match is
       the match at the position one notch further in the string.

       The additional state of being matched with zero-length is
       associated to the matched string, and is reset by each
       assignment to pos().