[sf-perl] Quick question on regular expression and substitution

[Loo, Peter # PHX]

Hi All,
 
Would some one know of a one liner that would replace the third
occurrence of a substring within a string?  For example:
 
$string = "?, ?, ?, ?";
 
I want to replace the third occurrence of "?" with "to_date(?,
'YYYY-MM-DD HH24:MI:SS')".
 
Thanks in advance.
 
Peter
 
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