#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "((?:[^.]*?\.){2}[^.]*)\.(.*)"
Local $sString = "My name is Resheil. I'm developing in PHP. I have used mysql for my application. I want to replace the full stop on the nth position in php."
Local $sSubst = "$1. Here I had a full stop.$2"
Local $sResult = StringRegExpReplace($sString, $sRegex, $sSubst)
MsgBox($MB_SYSTEMMODAL, "Result", $sResult)
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for AutoIt, please visit: https://www.autoitscript.com/autoit3/docs/functions/StringRegExp.htm