feat: add raw identifier syntax

CHANGES.md

@@ -3,6 +3,8 @@
 - rtop: read `~/.config/rtop/init.re` configuration file (@anmonteiro, [#2813])
     - the `-init FILE` flag works as before
 - rtop: ignore `~/.ocamlinit.ml` or `~/.config/utop/init.ml` config files (@anmonteiro, [#2813])
+- Add support for raw identifier syntax (@anmonteiro,
+  [#2796](https://github.com/reasonml/reason/pull/2796))
 
 ## 3.14.0
 

src/reason-parser/reason_declarative_lexer.mll

@@ -385,6 +385,7 @@ let hex_float_literal =
   (['p' 'P'] ['+' '-']? ['0'-'9'] ['0'-'9' '_']* )?
 
 let literal_modifier = ['G'-'Z' 'g'-'z']
+let raw_ident_escape = "\\#"
 
 rule token state = parse
   | "\\" newline {
@@ -408,6 +409,8 @@ rule token state = parse
     { QUESTION }
   | "=?"
     { set_lexeme_length lexbuf 1; EQUAL }
+  | raw_ident_escape (lowercase identchar * as name)
+      { LIDENT name }
   | lowercase identchar *
     { let s = Lexing.lexeme lexbuf in
       try Hashtbl.find keyword_table s
@@ -521,7 +524,7 @@ rule token state = parse
     { CHAR (char_for_decimal_code lexbuf 2) }
   | "'\\" 'x' ['0'-'9' 'a'-'f' 'A'-'F'] ['0'-'9' 'a'-'f' 'A'-'F'] "'"
     { CHAR (char_for_hexadecimal_code lexbuf 3) }
-  | "'" (("\\" _) as esc)
+  | "'" (("\\" [^ '#']) as esc)
     { raise_error (Location.curr lexbuf) (Illegal_escape esc);
       token state lexbuf
     }