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