JS - How to validate if string is a number

Question

I try to validate various input strings. Therefore I tried to use regular expressions. Unfortunately all negativ cases match to true as well. How can I sepcify that the regular expression has to match the whole string?

const regex = /(^([0-9]+(.||,)+[0-9]?)$)||(^([0-9]+)$)||(^(.||,)+[0-9]+$)/;

const testSetPositiv = ["34", "45.45", ".7867", "3434,34", ",7834", "546.", "45546,", "0"]

const testSetNegativ = ["wegrger", "3443,34erf", "3443.34erf", ",", ".", "4525tg", "1,211.23", "1.211,23"]


testSetPositiv.forEach(function(value) {
  console.log(regex.test(value));
  //should always return true
})
console.log("----------------");

testSetNegativ.forEach(function(value) {
  console.log(regex.test(value));
  //should always return false
})

I also tried to use parseFloat(...), but it accepts for example "5.45abc":

const testSetPositiv = ["34", "45.45", ".7867", "3434,34", ",7834", "546.", "45546,", "0"]

const testSetNegativ = ["wegrger", "3443,34erf", "3443.34erf", ",", ".", "4525tg", "1,211.23", "1.211,23"]


testSetPositiv.forEach(function(value) {
  console.log(parseFloat(value) >= -Number.MAX_VALUE)
  //should always return true

})
console.log("----------------");

testSetNegativ.forEach(function(value) {
  console.log(parseFloat(value) >= -Number.MAX_VALUE)

  //should always return false
})

@NinaScholz yes, should be valid in this case; e.g., "123,123" true, BUT separators shouldn't be accepted; e.g., "1.000,23" false, "1,211.23" false — d4rty
– d4rty, Commented May 26, 2018 at 16:28
are you doing this because the number values are all strings ? (the items in array with quotes will become strings). if the number values in the array did not have strings then they would be considered numbers. E.g. [4,66,"66","4"] is 2 numbers and 2 strings — J nui
– J nui, Commented May 26, 2018 at 16:35

Kosh · Accepted Answer · 2018-05-26 16:41:39Z

1

A simple regex solution:

const regex = /^(\d*[.,]?\d+|\d+[.,]?\d*)$/;

const testSetPositiv = ["34", "45.45", ".7867", "3434,34", ",7834", "546.", "45546,", "0"]

const testSetNegativ = ["wegrger", "3443,34erf", "3443.34erf", ",", ".", "4525tg", "1,211.23", "1.211,23"]



testSetPositiv.forEach(function(value) {
  console.log(regex.test(value));
  //should always return true
})
console.log("----------------");

testSetNegativ.forEach(function(value) {
  console.log(regex.test(value));
  //should always return false
})

answered May 26, 2018 at 16:41

Kosh

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Comments

Takit Isy · Accepted Answer · 2018-05-26 16:52:43Z

0

parseInt and parseFloat can return numbers even if the string contains letters, check snippet:

const testSetNegativ = ["wegrger", "3443,34erf", "3443.34erf", ",", ".", "4525tg", "1,211.23", "1.211,23"];
testSetNegativ.forEach(function(value) {
  console.log(parseFloat(value));
})

I suggest you to use:

.replace(',', '.') to replace the commas (used in Europe as decimal separator) with the correct JavaScript decimal separator,
Unary operator + to easily convert to number. (It will return NaN if not a number)

const testSetPositiv = ["34", "45.45", ".7867", "3434,34", ",7834", "546.", "45546,", "0"]
const testSetNegativ = ["wegrger", "3443,34erf", "3443.34erf", ",", ".", "4525tg", "1,211.23", "1.211,23"];

testSetPositiv.forEach(function(value) {
  console.log(+(value.replace(',', '.')) >= -Number.MAX_VALUE)
  //should always return true
})
console.log("----------------");
testSetNegativ.forEach(function(value) {
  console.log(+(value.replace(',', '.')) >= -Number.MAX_VALUE)
  //should always return false
})

Hope it helps.

edited May 26, 2018 at 16:52

answered May 26, 2018 at 16:32

Takit Isy

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8 Comments

d4rty Over a year ago

Sorry, I missed some negative examples, which might be not covered with this approach. I updated the question with th e negative examples; e.g., "1,211.23", "1.211,23" should be both evaluate to false

Takit Isy Over a year ago

@d4rty Are you using , as decimal or thousand separator? Anyway, I've updated my snippets!

d4rty Over a year ago

No they shouldn't be accepted (false input), any kinfd of thousand separator

Takit Isy Over a year ago

@d4rty If they shouldn't be accepted, why some of them are in the testSetPositiv ?

d4rty Over a year ago

Which string do you exaclty mean in the positive set?

|

My Nguyen · Accepted Answer · 2018-05-26 16:35:58Z

0

You can use the Number class to validate a string is a valid number. If it is not a valid number, it will return NaN

Example:

>Number(".23232")
0.23232
>Number(".23232abc")
NaN

answered May 26, 2018 at 16:35

My Nguyen

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Comments

J nui · Accepted Answer · 2018-05-26 16:41:30Z

0

use isNaN , (side note "45546," should not be a number)

        const testsetMixed = [34, 45.45, ".7867", "wegrger", "3443,34erf", "546.", "45546,", "0"]


    testsetMixed.forEach(function(value){
       if( isNaN(value)) {
         console.log('not number');
       } else {
         console.log('number');
       }

    })

answered May 26, 2018 at 16:41

J nui

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Comments

charlietfl · Accepted Answer · 2018-05-26 16:42:34Z

0

Can use isNan() which returns true if the input string is not numeric

answered May 26, 2018 at 16:42

charlietfl

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