PL/SQL: using INSTR and SUBSTR is a lot faster than REGEXP_SUBSTR

Given this "labels":
"InterfaceID=Common_NCRS;TechnicalMessageID=Common_NCRS^ACMEPreOrder.quote_order^5505352925343722746--4337ac89.1404f3a3d07.-8bc;EventType=InvokedACME;PathOfService=Commons_NCRS/ProxyServices/Common_NCRS_PS;EventOccuredOn=2013-08-05T18:17:59.470+02:00;BusinessID=DE11PC089^d3a59751-38e4-eed5-7bf7-4f45592a9d18;ServerName=osbpr1ms2" and this labelname "EventOccuredOn", the following function extracts the value "2013-08-05T18:17:59.470+02:00" :

create or replace 
FUNCTION ACME_findLabelValue 
  (labels IN VARCHAR2, labelname IN VARCHAR2 )  return VARCHAR2
is 
begin
  return  REPLACE(REGEXP_SUBSTR(labels, labelname || '=[^;]+'), labelname || '=', '');
end;

Performance was abysmal. I have replaced with this much faster function:

create or replace 
FUNCTION ACME_findLabelValue 
  (labels IN VARCHAR2, labelname IN VARCHAR2 )  return VARCHAR2
is 
v_delimpos1 PLS_INTEGER;
v_delimpos2 PLS_INTEGER;
labels2 VARCHAR2(4000);
begin

  v_delimpos1 := INSTR(labels, labelname || '=' );
  if v_delimpos1 > 0 then
    labels2 := SUBSTR(labels, v_delimpos1 + 1 + LENGTH(labelname));
    v_delimpos2 := INSTR(labels2, ';' );
    return SUBSTR(labels2, 1, v_delimpos2 - 1);  
  else
    return '';
  end if;
end;