The purpose of the _PPIx-Regexp_ package is to parse regular expressions in
a manner similar to the way the PPI package parses Perl. This class forms
the root of the parse tree, playing a role similar to PPI::Document.

This package shares with PPI the property of being round-trip safe. That
is,

my $expr = 's/ ( \d+ ) ( \D+ ) /$2$1/smxg';
my $re = PPIx::Regexp->new( $expr );
print $re->content() eq $expr ? "yes\n" : "no\n"

should print 'yes' for any valid regular expression.

Navigation is similar to that provided by PPI. That is to say, things like
'children', 'find_first', 'snext_sibling' and so on all work pretty much
the same way as in PPI.

The class hierarchy is also similar to PPI. Except for some utility classes
(the dumper, the lexer, and the tokenizer) all classes are descended from
PPIx::Regexp::Element, which provides basic navigation. Tokens are
descended from PPIx::Regexp::Token, which provides content. All containers
are descended from PPIx::Regexp::Node, which provides for children, and all
structure elements are descended from PPIx::Regexp::Structure, which
provides beginning and ending delimiters, and a type.

There are two features of PPI that this package does not provide -
mutability and operator overloading. There are no plans for serious
mutability, though something like PPI's 'prune' functionality might be
considered. Similarly there are no plans for operator overloading, which
appears to the author to represent a performance hit for little tangible
gain.