lexer: recognize a preprocessor directive with no trailing newline

The cpp lexer's PREPRO pattern (`^#.-[^\\]\n`) requires a literal trailing
newline, so a `#...` directive on the final line of the input without a
newline never matched. It fell through to the `^.` catch-all and was
tokenized as `#`, then the following words as identifiers, instead of a
single `prepro` token.

Add a PREPRO_EOF pattern (`^#.-[^\\]$`) tried right after PREPRO, so a
trailing directive at end-of-input is recognized while newline-terminated
and backslash-continued directives keep matching PREPRO first.

Fixes #450

Author: youdie006

lua/pl/lexer.lua

@@ -51,6 +51,9 @@ local CHAR1 = "^''"
 local CHAR2 = [[^'(\*)%1']]
 local CHAR3 = [[^'.-[^\](\*)%1']]
 local PREPRO = '^#.-[^\\]\n'
+-- A preprocessor directive on the final line may have no trailing newline;
+-- match it up to the end of the input so it is still recognized (issue #450).
+local PREPRO_EOF = '^#.-[^\\]$'
 
 local plain_matches,lua_matches,cpp_matches,lua_keyword,cpp_keyword
 
@@ -393,6 +396,7 @@ function lexer.cpp(s,filter,options)
         cpp_matches = {
             {WSPACE,wsdump},
             {PREPRO,pdump},
+            {PREPRO_EOF,pdump},
             {NUMBER3,ndump},
             {IDEN,cpp_vdump},
             {NUMBER4,ndump},

tests/test-lexer.lua

@@ -121,6 +121,12 @@ test_scan([['' "" " \\" '\'' "'"]], nil, nil, {
     {'string', "'"}
 }, 'cpp')
 
+-- A preprocessor directive on the final line without a trailing newline must
+-- still be recognized as a single 'prepro' token (issue #450).
+test_scan("#define ASDF", {}, nil, {
+    {'prepro', '#define ASDF'}
+}, 'cpp')
+
 local iter = lexer.lua([[
 foo
 bar