$string = '\u $';
#should become
#'username $'

#ideally it would be implimented w/ a loop/switch
while (#some magic) {
  switch ($1) {
    case "\u" { # $1 = $ENV{USER} }
    ...
  }
}

# or maybe a call to a sub

replace($str,
  "\u" => $ENV{USER}, # replace \u with USER env var
  "\f" => sub { 'foo' }, # replace \f with the result of sub
  "\r\{([^}]+)}" = > sub { reverse($_[1] }, # replace \r{...} with string in braces reversed
  "\(?:\d\d\d)" => sub { chr($_[1] } # replace 3 numbers with the corresponding character number
};