lexer and parser problem

I have a question about my program which is divided in two files: lexer.mll and parser.mly.
The first file is lexer.mll:

rule lex = parse
  | [' ' '\n' '\t'] { lex lexbuf }
  | ['0'-'9']+ as s { INT(int_of_string s) }
  | ['a'-'z' 'A'-'Z'] ['a'-'z' 'A'-'Z' '0'-'9']* as word { IDENT word }
  | '+'             { ADD } 
  | "plus"          { PLUS } 
  | eof             { EOF  }

The second file is parser.mly:

%{ type expr = 
Int of int
|String of string
| Add of expr * expr 

%}

%token <int> INT
%token <string> IDENT
%token ADD 
%token PLUS 
%token EOF


%start expr
%type <expr> expr

%left PLUS ADD 

%%

expr:                 
IDENT             { String $1 }
|INT              { Int $1 }
|expr PLUS expr   { Add($1, $3) }
|expr ADD expr    { Add($1, $3) }
;

So as you can see, for example I want to get the same results for both "1+2" and "1plus2" like :
Add (Int 1, Int 2) 
But I don't know why it works for "1+2" and it doesn't work for "1plus2".
I use these commands to run the program:

(*commands in Windows PowerShell*):
ocamllex lexer.mll
ocamlyacc parser.mly

(*commands and results in OCamlWinPlus*):

#use "parser.ml";;
#use "lexer.ml";;
# expr lex (Lexing.from_string "1+2");;
- : expr = Add (Int 1, Int 2)
# expr lex (Lexing.from_string "1plus2");;
Exception: Parsing.Parse_error.

Why it works correctly for 1+2 but not for 1plus2? and also how can I use a main class for this program which can get the inputs as a .txt file and return the result in another .txt file so I don't need to use this command (expr lex (Lexing.from_string "1plus2");;) for each code line?

Regards


 
--
Arash Alavi
Department of Computer Engineering & Information Technology
Amirkabir University of Technology
Email(s): [email protected] , [email protected] page: http://ceit.aut.ac.ir/~arash.alavi