$filename =~ s{ 
          ^ ~             # find a leading tilde 
          (               # save this in $1 
              [^/]        # a non-slash character 
                    *     # repeated 0 or more times (0 means me) 
          ) 
        }{ 
          $1 
              ? (getpwnam($1))[7] 
              : ( $ENV{HOME} || $ENV{LOGDIR} ) 
        }ex;