#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(?<=if \(|elseif )\$\{([^}]*)\}([+-])\$\{([^}]*)\}"
Local $sString = "<#if (${val1}-${val2}) + ${f1(${a}+${b}*${c})} > 2000>ABC<#elseif ${n1}+${n2}* ${f1(${n3}-${n4})} < 500>DEF</#if>"
Local $sSubst = "\1\2\3"
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