use YAPE::Regex::Explain;
print YAPE::Regex::Explain ->new(
    qr/\[(\/?)(lien|url|citation|img).+?\]/s
)->explain;
__END__
The regular expression:

(?s-imx:\[(/?)(lien|url|citation|img).+?\])

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?s-imx:                 group, but do not capture (with . matching
                         \n) (case-sensitive) (with ^ and $ matching
                         normally) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \[                       '['
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    /?                       '/' (optional (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    lien                     'lien'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    url                      'url'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    citation                 'citation'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    img                      'img'
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  .+?                      any character (1 or more times (matching
                           the least amount possible))
----------------------------------------------------------------------
  \]                       ']'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------