exactly.js

let formula = '(1 / (1 * 1))';
formula = '(1)';

class FormulaParser {
	constructor() {
		this.index = 0;
		this.lexemes = {
			delimiters: {
				tokens: '()',
				expect: [ 'identifiers', 'value' ]
			},
			operations: {
				tokens:  '+-*/',
				expect: ['value', 'identifiers']
			},
			identifiers: {
				tokens:  'abcdefghijklmnopqrstuvxyz',
				expect: ['operation', 'delimiters']
			},
			value: {
				tokens:  '0123456789' ,
				expect: ['operation', 'delimiter']
			}
		};

		this.states = ['scope']; 

		this.currentExpectation = ['delimiters', 'identifiers', 'value'];
	}

	parse(input) {	
		while(this.index <= input.length) {
			const current = input[this.index];

			if( this.match(current, this.lexemes.delimiters.tokens) && this.match('delimiters', this.currentExpectation) ) {
				this.currentExpectation = this.lexemes.delimiters.expect;
			}

			if( this.match(current, this.lexemes.identifiers.tokens) && this.match('identifiers', this.currentExpectation) ) {
				this.currentExpectation = this.lexemes.identifiers.expect;
			}

			if( this.match(current, this.lexemes.value.tokens) && this.match('value', this.currentExpectation) ) {
				this.currentExpectation = this.lexemes.value.expect;
			}

			this.index++
		}
	}

	match(input, token) {
		if (Array.isArray(token)){
			token = token.join('')
		}

		// const tokenArray = token.
		// console.log('IIIIIII', input.split(''), token)
		if (token.split('').find( function(index) { return input === index }) !== -1 ) {
			return true;
		}
		return false;
	}
}

const formulaParser = new FormulaParser();

formulaParser.parse(formula);