#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?i)^(.*)_[12]?\d{1,2}\.[12]?\d{1,2}\.[12]?\d{1,2}\.[12]?\d{1,2}_\d+\.([a-z]{3,4})$"
Local $sString = "pdf_of_filename_53613785621.183761237_127.10.10.0_8726347.xls"
Local $sSubst = "\1.\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