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.

The PPIx::Regexp object is instantiated using either new() or
new_from_cache(). Either way, you must pass it a regular expression,
either as a string or as one of the three relevant PPI objects:
PPI::Token::StringLike::Regexp, PPI::Token::Regexp::Match, or
PPI::Token::Regexp::Substitute. In the case of new_from_cache(), only
one PPIx::Regexp object will be generated from a given PPI object;
subsequent calls with the same PPI object will return the same
PPIx::Regexp object.