Oder ohne StrigRegExp:
[autoit]
$str = "abcd= 29876 xyz= 0" & @CRLF & "abcd= 123 xyz= 945" & @CRLF & "abcd= 4567 xyz= 345" & @CRLF & "abcd= 8902 xy"
$end = StringInStr($str, ' xyz=', 0, -1)
$start = StringInStr($str, 'abcd=', 0, -1, $end) + 6
$out = StringMid($str, $start, $end - $start)
MsgBox(0, 0, '"'&$out&'"')