PHP

function formatCnpjCpf($value)
{
  $CPF_LENGTH = 11;
  $cnpj_cpf = preg_replace("/\D/", '', $value);
  
  if (strlen($cnpj_cpf) === $CPF_LENGTH) {
    return preg_replace("/(\d{3})(\d{3})(\d{3})(\d{2})/", "\$1.\$2.\$3-\$4", $cnpj_cpf);
  } 
  
  return preg_replace("/(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})/", "\$1.\$2.\$3/\$4-\$5", $cnpj_cpf);
}

JS

function formatCnpjCpf(value) {
  const CPF_LENGTH = 11;
  const cnpjCpf = value.replace(/\D/g, '');
  
  if (cnpjCpf.length === CPF_LENGTH) {
    return cnpjCpf.replace(/(\d{3})(\d{3})(\d{3})(\d{2})/g, "\$1.\$2.\$3-\$4");
  } 
  
  return cnpjCpf.replace(/(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})/g, "\$1.\$2.\$3/\$4-\$5");
}

Oracle PL/SQL (by @claudioacioli)

create function formatCnpjCpf(value varchar2) return varchar2
is
  cnpj_cpf varchar2(14) := regexp_replace(value, '(\D)', ''); 
begin
  if length(cnpj_cpf) = 11 then
    return regexp_replace(cnpj_cpf, '(\d{3})(\d{3})(\d{3})(\d{2})','\1.\2.\3-\4');
  end if;  
  return regexp_replace(lpad(cnpj_cpf, 14, '0'),'(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})','\1.\2.\3/\4-\5');
end;