I see that you have already received good advice from other monks. I just wanted to mention that item 9 in Basic debugging checklist shows how to use the YAPE::Regex::Explain module to demystify regular expressions. For example, you can see an explanation of your regex by running this

perl -MYAPE::Regex::Explain -E 'say YAPE::Regex::Explain->new("(?:ftp:
+\/\/)?\/{5}(a-z_)\/")->explain()'
[download]

The output is

The regular expression:

(?-imsx:(?:ftp://)?/{5}(a-z_)/)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    ftp://                   'ftp://'
----------------------------------------------------------------------
  )?                       end of grouping
----------------------------------------------------------------------
  /{5}                     '/' (5 times)
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    a-z_                     'a-z_'
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  /                        '/'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]