#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(?:^<[^\n<>]+>\h+<[^<>\n]+>\h+"|\G(?!^))[^_\n]+\K_(?=[^"\n]*")"
Local $sString = "<http://dbpedia.org/resource/Animalia_(book)> <http://dbpedia.org/property/author> "Graeme_Base" ." & @CRLF & _
"<http://dbpedia.org/resource/Animalia_(book)> <http://dbpedia.org/property/illustrator> "Graeme_Base" ." & @CRLF & _
"<http://dbpedia.org/resource/Animalia_(book)> <http://dbpedia.org/property/illustrator> "Graeme_Base_test1_test2" ."
Local $sSubst = " "
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