#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)^[A-Z]\.(?=\s+[a-z]+\s*(?:[A-Z]\.\s+[a-z]+\s*)*([A-Z]+)\s*$)"
Local $sString = "P. salinarum" & @CRLF & _
"P. montana" & @CRLF & _
"P. conspicua" & @CRLF & _
"PLATESSA" & @CRLF & _
"A. cocconeiformis" & @CRLF & _
"A. coarctata" & @CRLF & _
"A. brevipes" & @CRLF & _
"ACHNANTHES"
Local $sSubst = "$1"
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