#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(a\s)(?:target=".*?"\s)?"
Local $sString = "<a target="_blank" href="https://vk.com/"></a>" & @CRLF & _
"<a href="/list"></a>" & @CRLF & _
"<a href="https://vk.com/"></a>" & @CRLF & _
"<a target="_self" href="https://vk.com/"></a>"
Local $sSubst = "$1target="_blank" "
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