MyFile := "file.txt"
FileAppend, `naaa, % MyFile
FileRead, filetext, % MyFile
test := "aaa"
res := RegExMatch(filetext, "m`a)^\Q" . test . "\E$")
MsgBox, % "Строка " . (res ? "" : "не ") . "найдена"

test := "aa"
res := RegExMatch(filetext, "m`a)^\Q" . test . "\E$")
MsgBox, % "Строка " . (res ? "" : "не ") . "найдена"
Return   

MyFile = file.txt
	FileRead, filetext, %MyFile%
	AAA = %Clipboard%
	IfInString, filetext, %AAA%
	{
		MsgBox, It'sInFile
	} Else {
		FileAppend, % (filetext="" ? "" : "`n") . AAA, %MyFile%
		MsgBox, NowInFile
	}
	Return