#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(<xref>[^\d]+)(\d)(<\/xref)"
Local $sString = "This is a example string and first line with <xref>id1</xref>then,<xref>id2</xref>and with no line breaks<xref>id3</xref>. This is a second line which has <xref>id4</xref>"
Local $sSubst = "$1eqvalue$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