#include <StringConstants.au3> ; to declare the Constants of StringRegExp
#include <Array.au3> ; UDF needed for _ArrayDisplay and _ArrayConcatenate
Local $sRegex = "(?m)[^.]+_([^.]+\.[^.]+)"
Local $sString = ""1234_4534_41247612_2462184_2131_ABCDEF.GHI.xlsx"" & @CRLF & _
""1234_4534__sfhaksj_DHJKhd_hJD_41247612_2462184_2131_PQRST.GHI.xlsx"" & @CRLF & _
""12JSAF34_45aAF34__sfhaksj_DHJKhd_hJD_41247612_2f462184_2131_JKLMN.OPQ.xlsx"" & @CRLF & _
""1234_4534__sfhaksj_DHJKhd_hJD_41FA247612_2462184_2131_WXY.TUV.xlsx""
Local $aArray = StringRegExp($sString, $sRegex, $STR_REGEXPARRAYGLOBALFULLMATCH)
Local $aFullArray[0]
For $i = 0 To UBound($aArray) -1
_ArrayConcatenate($aFullArray, $aArray[$i])
Next
$aArray = $aFullArray
; Present the entire match result
_ArrayDisplay($aArray, "Result")
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