Função para formatar CNPJ e CPF, disponível em PHP e JS

formatar_cnpj_cpf.md

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;

PostgreSQL

CREATE OR REPLACE FUNCTION public.formatCpfCnpj(cpfcnpj character varying)
 RETURNS character varying
 LANGUAGE plpgsql
AS $function$
	DECLARE
		cnpjcpf_only_numbers varchar;
	BEGIN
		cnpjcpf_only_numbers = regexp_replace(cpfcnpj, '(\D)', ''); -- Remove todos os caracteres, exceto números
		case length(cnpjcpf_only_numbers)
			when 11 then
				return regexp_replace(cnpjcpf_only_numbers, '(\d{3})(\d{3})(\d{3})(\d{2})','\1.\2.\3-\4');
			when 14 then
				return regexp_replace(cnpjcpf_only_numbers, '(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})','\1.\2.\3/\4-\5');
			else
				return cnpjcpf_only_numbers;
		END case;
	END;
$function$
;