expressionparser

Expression Parser

A no-dependency, easy-to-extend interpreter for parsing and evaluating expressions.

This project is just a single file, aiming to be easy to understand, extend, and build upon.

Install

npm install expressionparser

Built-in Languages

A built-in language called "formula" is provided as the default.

Evaluating an expression:

import { init, formula } from 'expressionparser'

const parser = init(formula, (term: string) => {
  if (term === "MY_VARIABLE") {
    return 42;
  } else {
    throw new Error(`Invalid term: ${term}`);
  }
});

parser.expressionToValue("(1 + 1) + 40 = MY_VARIABLE"); // true

Custom Language

Parse simple expressions, in a language of your own description, such as:

(A AND B) OR NOT (C OR D)

or

1 + 1 * 2 - (10 / 2) + SQRT 16

The latter language can be configured as follows:

const arithmeticLanguage = {
  INFIX_OPS: {
    '+': function(a, b) {
      return a + b;
    },
    '-': function(a, b) {
      return a - b;
    },
    '*': function(a, b) {
      return a * b;
    },
    '/': function(a, b) {
      return a / b;
    },
    ',': function(a, b) {
      return [a] + b;
    }
  },
  PREFIX_OPS: {
    'SQRT': function(expr) {
      return Math.sqrt(expr);
    },
    'POW': function(expr) {
      return Math.pow(expr[0], expr[1]);
    }
  },
  PRECEDENCE: [['SQRT', 'POW'], ['*', '/'], ['+', '-'], [',']],
  GROUP_OPEN: '(',
  GROUP_CLOSE: ')',
  SEPARATORS: [','],
  WHITESPACE_CHARS: [" "],
  SYMBOLS: ['(', ')', '+', '-', '*', '/', ','],

  termDelegate: function(term) {
    return parseInt(term);
  }
};

and evaluated as: const expr = 'pow(1 + 1 * 2 - (10 / 2) + sqrt(16), 2)'.toUpperCase(); const result = new ExpressionParser(arithmeticLanguage).expressionToValue(expr);

(which will result in 4)

Tokeniser

This uses the built-in tokeniser (which is very simple), but you can write your own tokeniser (e.g. for differentiating between prefix minus and infix minus) and pass the tokens into:

const result = new ExpressionParser(arithmeticLanguage).evaluateTokens(['1', '+', '1']);

(which will also result in 2)

RPN

This parser will also convert between an expression and a Reverse Polish Notation (RPN) list:

const parser = new ExpressionParser();

parser.expressionToRpn(expr); // returns RPN list
parser.tokensToRpn(exprTokenList); // returns RPN list
parser.rpnToExpression(rpnList); // returns expression string
parser.rpnToTokens(rpnList); // returns expression token list

Further configuration

The SEPARATORS, WHITESPACE_CHARS, and SYMBOLS options are to assist the built-in tokeniser and expression string builder. By default terms used to define operators or terms are strings of characters separated by any member of WHITESPACE_CHARS. Characters in SYMBOLS will be collected as a separate category to any other characters.

e.g. If '!' is in SYMBOLS, with ' ' in WHITESPACE_CHARS, we can use "!!A" instead of "!! A" to provide the tokenization of ["!!", "A"]

SEPARATORS are single-character symbols which are self-terminating (they stand on their own).

e.g. If '!' is instead in SEPARATORS, "!!A" will be tokenized to: ["!", "!", "A"]

The termDelegate option is used to evaluate the terminal symbols in the parse tree (i.e. the values).