Possible bug in the standard lexer

demo.ipynb

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "import lark"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[Token(ASSIGNMENT_OP, '+=')]"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "grammar = r'''\n",
    "start: BINOP | ASSIGNMENT_OP\n",
    "\n",
    "ASSIGNMENT_OP: \"=\"\n",
    "    | \"+=\"\n",
    "\n",
    "BINOP: \"*\"\n",
    "    | \"/\"\n",
    "    | \"%\" \n",
    "    | \"+\"\n",
    "    | \"-\"\n",
    "\n",
    "%import common.WS\n",
    "%ignore WS\n",
    "'''\n",
    "\n",
    "parser = lark.Lark(grammar, parser='lalr')\n",
    "code = '+='\n",
    "list(parser.lex(code))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[Token(BINOP, '+'), Token(ASSIGNMENT_OP, '=')]"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "grammar = r'''\n",
    "start: BINOP | ASSIGNMENT_OP\n",
    "\n",
    "ASSIGNMENT_OP: \"=\"\n",
    "    | \"+=\"\n",
    "\n",
    "BINOP: \"*\"\n",
    "    | \"/\"\n",
    "    | \"%\" \n",
    "    | \"+\"\n",
    "    | \"-\"\n",
    "    | \"long_operator\"\n",
    "\n",
    "%import common.WS\n",
    "%ignore WS\n",
    "'''\n",
    "\n",
    "parser = lark.Lark(grammar, parser='lalr')\n",
    "code = '+='\n",
    "list(parser.lex(code))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[Token(ASSIGNMENT_OP, '+=')]"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "grammar = r'''\n",
    "start: BINOP | ASSIGNMENT_OP\n",
    "\n",
    "ASSIGNMENT_OP: \"=\"\n",
    "    | \"+=\"\n",
    "    | \"even_longer_operator\"\n",
    "\n",
    "BINOP: \"*\"\n",
    "    | \"/\"\n",
    "    | \"%\" \n",
    "    | \"+\"\n",
    "    | \"-\"\n",
    "    | \"long_operator\"\n",
    "\n",
    "%import common.WS\n",
    "%ignore WS\n",
    "'''\n",
    "\n",
    "parser = lark.Lark(grammar, parser='lalr')\n",
    "code = '+='\n",
    "list(parser.lex(code))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Expected results**: `[Token(ASSIGNMENT_OP, '+=')]` for all of these code snippets\n",
    "\n",
    "**Actual results**: `[Token(ASSIGNMENT_OP, '+=')]`, `[Token(BINOP, '+'), Token(ASSIGNMENT_OP, '=')]`, `[Token(ASSIGNMENT_OP, '+=')]`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.7.3"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}