perl -MYAPE::Regex::Explain -e 'print YAPE::Regex::Explain->new(q/^(?:
+([smtwhfa])(?!.*\1))*$/)->explain'
The regular expression:

(?-imsx:^(?:([smtwhfa])(?!.*\1))*$)

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
----------------------------------------------------------------------
  (?:                      group, but do not capture (0 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
      [smtwhfa]                any character of: 's', 'm', 't', 'w',
                               'h', 'f', 'a'
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      .*                       any character except \n (0 or more
                               times (matching the most amount
                               possible))
----------------------------------------------------------------------
      \1                       what was matched by capture \1
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
  )*                       end of grouping
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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