$ perl -MYAPE::Regex::Explain -e " print YAPE::Regex::Explain->new( qr{\A ipv6 | alt \Z}imxs )->explain "
The regular expression:

(?imsx:\A ipv6 | alt \Z)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?imsx:                  group, but do not capture (case-insensitive)
                         (with ^ and $ matching start and end of
                         line) (with . matching \n) (disregarding
                         whitespace and comments):
----------------------------------------------------------------------
  \A                       the beginning of the string
----------------------------------------------------------------------
  ipv6                     'ipv6'
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  alt                      'alt'
----------------------------------------------------------------------
  \Z                       before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

##</code><code>##

[Visual of regex at 'rxrx' line 0]         [step: 5]

    \A                    Match only if at start of string

    ipv6                  Match a literal sequence ("ipv6")

  |                     Or...

    alt                   Match a literal sequence ("alt")

    \Z                    Match only if at end of string (or final newline)