you already received good hints. A minimal read of the manual is something to do.

Here the explanation of your regex:

perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new(qr/^[5
+]+$/)->explain()"

The regular expression:

(?-imsx:^[5]+$)

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):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  [5]+                     any character of: '5' (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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