function getstr(Ystr,Mstr:string;istar:byte):string; //从字符串中截取字符串 delphitop.com
var s:string;
i:byte;
begin
result:='';
i:=0;
for i:=0 to length(Ystr) do
if idelete(Ystr,1,pos(Mstr,Ystr)) else
break;
s:=copy(Ystr,i-strtoint(form1.edit4.text)-1,pos(Mstr,Ystr)-1);
delete(s,1,length(Mstr)-1);
if s<>'' then
result:=s else
result:='提取串为空串！'
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
edit2.Text:=getstr(edit1.Text,edit3.text,strtoint(edit4.text));
end;

procedure TForm1.Edit4KeyPress(Sender: TObject; var Key: Char);
begin
if (Key < '0') or (Key > '9')
then Key := Chr(0);
end;