I have been working on a function to parse a formula for some time, but haven't been able to make it work properly. It seems to not always work - it filters some parts of the text but not all.

parseFormula(e) {
    var formula = e.value, value = 0.00, tValue = 0.00, tFormula = '', dObj = {};
    if(formula !== undefined && formula !== "") {
        dObj._formulaIn = formula;
        var f = formula.split(/\s/g);    

        for(var i = 0; i < f.length; i++) {
            tFormula = f[i];
            // Replacing PI
            tFormula = tFormula.replace(/(pi)/gi,Math.PI);
            dObj._1_pi_done = tFormula;

            // Replacing Squareroot with placeholder
            tFormula = tFormula.replace(/(sqrt)/gi,"__sqrt__");
            tFormula = tFormula.replace(/(sqr)/gi,"__sqrt__");
            tFormula = tFormula.replace(/(kvrt)/gi,"__sqrt__");
            tFormula = tFormula.replace(/(kvr)/gi,"__sqrt__");
            dObj._2_sqrt_done = tFormula;

            // Removing units that may cause trouble
            tFormula = tFormula.replace(/(m2||m3||t2||t3||c2||c3)/gi,"");
            dObj._3_units_done = tFormula;

            // Removing text
            tFormula = tFormula.replace(/\D+[^\*\/\+\-]+[^\,\.]/gi,"");
            dObj._4_text_done = tFormula;

            // Removing language specific decimals
            if(Language.defaultLang === "no_NB") {
                tFormula = tFormula.replace(/(\.)/gi,"");
                tFormula = tFormula.replace(/(\,)/gi,".");
            } else {
                tFormula = tFormula.replace(/(\,)/gi,"");           
            }
            dObj._5_lang_done = tFormula;

            // Re-applying Squareroot
            tFormula = tFormula.replace(/(__sqrt__)/g,"Math.sqrt");
            dObj._6_sqrt_done = tFormula;

            if(tFormula === "") {
                f.splice(i,1);
            } else {
                f[i] = tFormula;
            }
            dObj._7_splice_done = tFormula;
            console.log(dObj);
        }

        formula = "";
        for(var j = 0; j < f.length; j++) {
            formula += f[j];   
        }

        try {
            value = eval(formula);
        } 
        catch(err) {}
        return value === 0 ? 0 : value.toFixed(4);
    } else {
        return 0;
    }
}

I am not sure about any of the RegEx used in this function, hence why I am asking for help. For example, I am not sure if /(pi)/ is the right way to get the exact text "pi" and replace it with 3.141.

(I am using eval at the moment, but it's merely used for development)

Any help appreciated.

Edit:

The Formula I am trying to parse is a user input formula. Where he/she would type something like: 2/0.6 pcs of foo * pi bar + sqrt(4) foobar. Where I would want it to strip all the non-math letters and calculate the rest. Meaning the above formula would be interpreted as (2/0.6) * 3.141 + Math.sqrt(4) => 12.47

Edit 2:

e is a ExtJS object, passed through by a field in a grid, it contains the following variables:

• colIdx (int)
• column (Ext.grid.column.Column)
• field (string)
• grid (Ext.grid.Panel)
• originalValue (string)
• record (Ext.data.Model)
• row (css selector)
• rowIdx (int)
• store (Ext.data.Store)
• value (string)
• view (Ext.grid.View)

Am currently unable to get the JSFiddle to work properly.

I have been working on a function to parse a formula for some time, but haven't been able to make it work properly. It seems to not always work - it filters some parts of the text but not all.

parseFormula(e) {
    var formula = e.value, value = 0.00, tValue = 0.00, tFormula = '', dObj = {};
    if(formula !== undefined && formula !== "") {
        dObj._formulaIn = formula;
        var f = formula.split(/\s/g);    

        for(var i = 0; i < f.length; i++) {
            tFormula = f[i];
            // Replacing PI
            tFormula = tFormula.replace(/(pi)/gi,Math.PI);
            dObj._1_pi_done = tFormula;

            // Replacing Squareroot with placeholder
            tFormula = tFormula.replace(/(sqrt)/gi,"__sqrt__");
            tFormula = tFormula.replace(/(sqr)/gi,"__sqrt__");
            tFormula = tFormula.replace(/(kvrt)/gi,"__sqrt__");
            tFormula = tFormula.replace(/(kvr)/gi,"__sqrt__");
            dObj._2_sqrt_done = tFormula;

            // Removing units that may cause trouble
            tFormula = tFormula.replace(/(m2||m3||t2||t3||c2||c3)/gi,"");
            dObj._3_units_done = tFormula;

            // Removing text
            tFormula = tFormula.replace(/\D+[^\*\/\+\-]+[^\,\.]/gi,"");
            dObj._4_text_done = tFormula;

            // Removing language specific decimals
            if(Language.defaultLang === "no_NB") {
                tFormula = tFormula.replace(/(\.)/gi,"");
                tFormula = tFormula.replace(/(\,)/gi,".");
            } else {
                tFormula = tFormula.replace(/(\,)/gi,"");           
            }
            dObj._5_lang_done = tFormula;

            // Re-applying Squareroot
            tFormula = tFormula.replace(/(__sqrt__)/g,"Math.sqrt");
            dObj._6_sqrt_done = tFormula;

            if(tFormula === "") {
                f.splice(i,1);
            } else {
                f[i] = tFormula;
            }
            dObj._7_splice_done = tFormula;
            console.log(dObj);
        }

        formula = "";
        for(var j = 0; j < f.length; j++) {
            formula += f[j];   
        }

        try {
            value = eval(formula);
        } 
        catch(err) {}
        return value === 0 ? 0 : value.toFixed(4);
    } else {
        return 0;
    }
}

I am not sure about any of the RegEx used in this function, hence why I am asking for help. For example, I am not sure if /(pi)/ is the right way to get the exact text "pi" and replace it with 3.141.

(I am using eval at the moment, but it's merely used for development)

Any help appreciated.

Edit:

The Formula I am trying to parse is a user input formula. Where he/she would type something like: 2/0.6 pcs of foo * pi bar + sqrt(4) foobar. Where I would want it to strip all the non-math letters and calculate the rest. Meaning the above formula would be interpreted as (2/0.6) * 3.141 + Math.sqrt(4) => 12.47

Edit 2:

e is a ExtJS object, passed through by a field in a grid, it contains the following variables:

• colIdx (int)
• column (Ext.grid.column.Column)
• field (string)
• grid (Ext.grid.Panel)
• originalValue (string)
• record (Ext.data.Model)
• row (css selector)
• rowIdx (int)
• store (Ext.data.Store)
• value (string)
• view (Ext.grid.View)

Am currently unable to get the JSFiddle to work properly.

• javascript
• regex

Share Improve this question Follow edited Aug 27, 2013 at 9:36 GauteR asked Aug 27, 2013 at 7:45 GauteRGauteR 36077 silver badges2828 bronze badges 10

• What is the question? – mishik Commented Aug 27, 2013 at 7:47
• 1 What does this have to do with Ext JS? – Evan Trimboli Commented Aug 27, 2013 at 7:48
• I am wondering what I'm doing wrong in my RegExs - if there is any obvious mistakes. – GauteR Commented Aug 27, 2013 at 7:49
• It's a function I use in ExtJS 4.2.x - for evaluating a field in the grid. – GauteR Commented Aug 27, 2013 at 7:50
• 1 A suggestion: add to your question an example formula you would like to parse. Without it it is very hard to validate the regexs you are trying to use. This might help everyone understand better what you want to acplish and will get you more help. – Joum Commented Aug 27, 2013 at 7:59

 |  Show 5 more ments

2 Answers 2

Sorted by: Reset to default 6

It's probably easier to tokenize the expression you want to parse. When tokenized it's way easier to read that stream of tokens and build your own expressions.

I've put up a demo on jsFiddle which can parse your given formula

In the demo I used this Tokenizer class and tokens to build a TokenStream from the formula.

function Tokenizer() {
    this.tokens = {};
    // The regular expression which matches a token per group.
    this.regex = null;
    // Holds the names of the tokens. Index matches group. See buildExpression()
    this.tokenNames = [];
}

Tokenizer.prototype = {
    addToken: function(name, expression) {
        this.tokens[name] = expression;
    },

    tokenize: function(data) {
        this.buildExpression(data);
        var tokens = this.findTokens(data);
        return new TokenStream(tokens);
    },

    buildExpression: function (data) {
        var tokenRegex = [];
        for (var tokenName in this.tokens) {
            this.tokenNames.push(tokenName);
            tokenRegex.push('('+this.tokens[tokenName]+')');
        }

        this.regex = new RegExp(tokenRegex.join('|'), 'g');
    },

    findTokens: function(data) {
        var tokens = [];
        var match;

        while ((match = this.regex.exec(data)) !== null) {
            if (match == undefined) {
                continue;
            }

            for (var group = 1; group < match.length; group++) {
                if (!match[group]) continue;

                tokens.push({
                    name: this.tokenNames[group - 1],
                    data: match[group]
                });
            }
        }

        return tokens;
    }
}


TokenStream = function (tokens) {
    this.cursor = 0;
    this.tokens = tokens;
}
TokenStream.prototype = {
    next: function () {
        return this.tokens[this.cursor++];
    },
    peek: function (direction) {
        if (direction === undefined) {
            direction = 0;
        }

        return this.tokens[this.cursor + direction];
    }
}

Defined tokens

tokenizer.addToken('whitespace', '\\s+');
tokenizer.addToken('l_paren', '\\(');
tokenizer.addToken('r_paren', '\\)');
tokenizer.addToken('float', '[0-9]+\\.[0-9]+');
tokenizer.addToken('int', '[0-9]+');
tokenizer.addToken('div', '\\/');
tokenizer.addToken('mul', '\\*');
tokenizer.addToken('add', '\\+');
tokenizer.addToken('constant', 'pi|PI');
tokenizer.addToken('id', '[a-zA-Z_][a-zA-Z0-9_]*');

With the above tokens defined the tokenizer can recognize everything in your formula. When the formula

2/0.6 pcs of foo * pi bar + sqrt(4) foobar

is tokenized the result would be a token stream similar to

int(2), div(/), float(0.6), whitespace( ), id(pcs), whitespace( ), id(of), whitespace( ), id(foo), whitespace( ), mul(*), whitespace( ), constant(pi), whitespace( ), id(bar), whitespace( ), add(+), whitespace( ), id(sqrt), l_paren((), int(4), r_paren()), whitespace( ), id(foobar)

You cannot really use a regular expression to match a formula. Formulae are a context-free language and regular expressions are limited to regular languages, the latter being a subset of the former. There are a number of algorithms for recognizing context-free languages such as CYK and LL parsers. I don't remend studying those if you already haven't since the topic is quite large.

What you can do quickly, efficiently and easy though, is to attempt to calculate the formula using Reverse Polish Notation (RPN) (use the Shunting Yard algorithm to convert your formula to RPN). If the attempt fails (due to parenthesis not maching, invalid functions / constants, w/e), clearly the text is not a formula, otherwise all is good. Shunting yard is not a particularly difficult algorithm and you should have no trouble implementing it. Even if you do, the wikipedia page I linked above has pseudo code and there a good number of questions in SO as well to help you.