parseCall.js

tokenRE = /^[A-Za-z_]\w{0,31}/; 
numeralRE = /^\d+/; 

function matchAndReturn(str, re) { 
    var r = str.match(re); 
    return r? r[0]: null; 
} 

isToken = s => matchAndReturn(s, tokenRE); 
isNumeral = s => matchAndReturn(s, numeralRE); 

function parse(expr) { 
    var r = parseExpr(expr, 0); 
    return r? r[0]: null; 
} 

function skipBlank(expr, at) { 
    while(expr[at] === ' ') 
        at++; 
    return at; 
} 

function parseExpr(expr, at) { 
    at = skipBlank(expr, at); 
    
    //literal 
    var r = isNumeral(expr.substr(at)); 
    if(r) { 
        at += r.length; 
        return [parseInt(r), at]; 
    } 
    
    //call 
    return parseCall(expr, at); 
    
} 

function parseCall(expr, at) { 
    var arr = []; 
    
    //operator 
    at = skipBlank(expr, at); 
    var r = isToken(expr.substr(at)); 
    if(!r) return null; 
    arr.push(r); 
    at += r.length; 
    
    
    //'(' 
    at = skipBlank(expr, at); 
    if(expr[at] != '(') 
        return null; 
    at += 1; 
    
    //operand 1 
    at = skipBlank(expr, at); 
    r = parseExpr(expr, at); 
    if(!r) return null; 
    at = r[1]; 
    arr.push(r[0]); 
    
    //other operands 
    while(true) { 
        //',' 
        at = skipBlank(expr, at); 
        if(expr[at] != ',') 
            break; 
        at += 1; 
        
        //operand 
        at = skipBlank(expr, at); 
        r = parseExpr(expr, at); 
        if(!r) return null; 
        at = r[1]; 
        arr.push(r[0]); 
    } 
    
    //')' 
    at = skipBlank(expr, at); 
    if(expr[at] != ')') 
        return null; 
    at += 1; 
    
    return [arr, at]; 
}

parse("mul(add(sub(1, 2), 3), 4)");