function ExtrStrN(s,d:string;n:word):string;
var i,j,o,r:word; t:string;
begin
result:='';
if pos(d,s)=0 then exit;
if pos(d,s)=1 then s:=copy(s,length(d),length(s));
t:=s;r:=1;
for i:=1 to n do begin
o:=pos(d,t);
if o=0 then exit;
result:='';
for j:=r to o-1 do result:=result+t[j];
r:=o+length(d);
for j:=o to r-1 do t[j]:=char(1);
end;
end;

s1:=ExtrStrN('parameter1:::parameter2:::parameter3:::parameter4:::',':::',1);