JavaScript - Matching alphanumeric patterns with RegExp

Question

I'm new to RegExp and to JS in general (Coming from Python), so this might be an easy question: I'm trying to code an algebraic calculator in Javascript that receives an algebraic equation as a string, e.g.,

string = 'x^2 + 30x -12 = 4x^2 - 12x + 30';

The algorithm is already able to break the string in a single list, with all values on the right side multiplied by -1 so I can equate it all to 0, however, one of the steps to solve the equation involves creating a hashtable/dictionary, having the variable as key. The string above results in a list eq:

eq = ['x^2', '+30x', '-12', '-4x^2', '+12x', '-30'];

I'm currently planning on iterating through this list, and using RegExp to identify both variables and the respective multiplier, so I can create a hashTable/Dictionary that will allow me to simplify the equation, such as this one:

hashTable = {
    'x^2': [1, -4],
    'x': [30, 12],
    ' ': [-12]
}

I plan on using some kind of for loop to iter through the array, and applying a match on each string to get the values I need, but I'm quite frankly, stumped. I have already used RegExp to separate the string into the individual parts of the equation and to remove eventual spaces, but I can't imagine a way to separate -4 from x^2 in '-4x^2'.

duplicate stackoverflow.com/questions/388996/…

Chris Hawkes
– Chris Hawkes

2016-03-15 13:37:21 +00:00
Commented Mar 15, 2016 at 13:37 — Chris Hawkes
– Chris Hawkes, Commented Mar 15, 2016 at 13:37

JulCh · Accepted Answer · 2016-03-15 13:47:50Z

1

You can try this (-?\d+)x\^\d+.

When you execute match function :

var res = "-4x^2".match(/(-?\d+)x\^\d+/)

You will get res as an array : [ "-4x^2", "-4" ] You have your '-4' in res[1].

By adding another group on the second \d+ (numeric char), you can retrieve the x power.

var res = "-4x^2".match(/(-?\d+)x\^(\d+)/) //res = [ "-4x^2", "-4", "2" ]

Hope it helps

answered Mar 15, 2016 at 13:47

JulCh

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Jry9972 · Accepted Answer · 2016-03-15 13:40:17Z

0

If you know that the LHS of the hashtable is going to be at the end of the string. Lets say '4x', x is at the end or '-4x^2' where x^2 is at end, then we can get the number of the expression:

var exp = '-4x^2'
exp.split('x^2')[0] // will return -4

I hope this is what you were looking for.

answered Mar 15, 2016 at 13:40

Jry9972

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1 Comment

Matheus Avila Over a year ago

Yes, it did help a lot. I'll probably still use RegExp to discover which is the largest exponent in the equation to create the split parameter dynamically, so the code can be reused with ternary equations and above too.

Lukasz Wiktor · Accepted Answer · 2016-03-15 14:27:20Z

0

function splitTerm(term) {
    var regex = /([+-]?)([0-9]*)?([a-z](\^[0-9]+)?)?/
    var match = regex.exec(term);
    return {
        constant: parseInt((match[1] || '') + (match[2] || 1)),
        variable: match[3]
    }
}

splitTerm('x^2');  // => {constant:   1, variable: "x^2"}
splitTerm('+30x'); // => {constant:  30, variable: "x"}
splitTerm('-12');  // => {constant: -12, variable: undefined}

Additionally, these tool may help you analyze and understand regular expressions:

edited Mar 15, 2016 at 14:27

answered Mar 15, 2016 at 13:51

Lukasz Wiktor

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