#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?![^#]*#[^#]*#)\b1([a-z])(?=[^#]*#)"
Local $sString = "As a 1st step, you can rewrite #21x*x - 1y = 1z + 1#"
Local $sSubst = "$1"
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