#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "^(?:[^,]+,\s+){9}\K[^,]+"
Local $sString = "value1, value2, value3, value4, value5, value6, value7, value8, value9, value10"
Local $sSubst = "x"
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