求这样一个字符串函数？？？

[kevin_hz]

比如说

PFP_00316_00

有这样一个字符串

我只想将 第三个P 替换成D
别的P不替换

[zorou_fatal]

<%
contents="PFP_0P316_00"
startplace=1
for i=1 to 3
startplace=instr(startplace,contents,"p",1)
if startplace<>0 then
startplace=startplace+1
end if
next

contents=left(contents,startplace)+replace(contents,"P","D",startplace-1)
response.write contents
%>

[changechange]

<%
contents="PFP_0P316_00"
startplace=1
for i=1 to 3
startplace=instr(startplace,contents,"p",1)
if startplace<>0 then
startplace=startplace+1
end if
next

contents=left(contents,startplace)+replace(contents,"P","D",startplace-1)
response.write contents
%>