Lexer

This library can be used to generate tokens from the source code string. This will take Rules, source code as input and generates tokens. Tokenization is the first part of writing compilers.

You can extract tokens one by one by using next() function or you can toke use tokens() function to extract all tokens as array.

Installation

npm install @msnraju/lexer

API

Lexer class

This is the class to extract tokens.

• constructor(rules: Array, input: string)

  Lexer takes rules, input as arguments. details are in Usage section.

• next() : IToken

  returns the next token. If lexer reached end of the stream, it will return Token with type 'EOF'

• tokens(): IToken[]

  This static method returns all available tokens as array.

Lex Rule interface

Rule is a simple regular expression with a name.

• name

  Name of the rule, this will be assigned to token type

• expression

  Regular Expression to extract the token value

RuleName enumeration

This can be used as Rule Name for the following purposes.

• RuleName.SKIP

  Ignores the token

• RuleName.ERROR Throws runtime error.

Usage

The following sample code to extract tokens using lex rules.

import { Lexer, ILexRule, RuleName } from '@msnraju/lexer';

const rules: Array<ILexRule> = [
  { name: 'VALUE', expression: /[a-zA-Z_]*/ },
  { name: 'EQUALS', expression: /=/ },
  { name: 'PLUS', expression: /\+/ },
  { name: 'NUMBER', expression: /[0-9]+/ },
  { name: 'SEMICOLON', expression: /;/ },
  { name: RuleName.SKIP, expression: /[ \t\n]/ },
  { name: RuleName.ERROR, expression: /./ },
];

const tokens = Lexer.tokens(
  rules,
  `
first = 1;
second = 2;
sum = first + second;
`
);

const tokens = Lexer.tokens(rules, code);
console.log(JSON.stringify(tokens, null));

Output

    [
      {
        "value": "first",
        "type": "VALUE"
      },
      {
        "value": "=",
        "type": "EQUALS"
      },
      {
        "value": "1",
        "type": "NUMBER"
      },
      {
        "value": ";",
        "type": "SEMICOLON"
      },
      ...
      {
        "value": "sum",
        "type": "VALUE"
      },
      {
        "value": "=",
        "type": "EQUALS"
      },
      {
        "value": "first",
        "type": "VALUE"
      },
      {
        "value": "+",
        "type": "PLUS"
      },
      {
        "value": "second",
        "type": "VALUE"
      },
      {
        "value": ";",
        "type": "SEMICOLON"
      }
    ]