Count number of matches of a regex in Javascript

Question

I wanted to write a regex to count the number of spaces/tabs/newline in a chunk of text. So I naively wrote the following:-

numSpaces : function(text) { 
    return text.match(/\s/).length; 
}

For some unknown reasons it always returns 1 . What is the problem with the above statement? I have since solved the problem with the following:-

numSpaces : function(text) { 
    return (text.split(/\s/).length -1); 
}

Answer 1

tl;dr: Generic Pattern Counter

// THIS IS WHAT YOU NEED
const count = (str) => {
  const re = /YOUR_PATTERN_HERE/g
  return ((str || '').match(re) || []).length
}

For those that arrived here looking for a generic way to count the number of occurrences of a regex pattern in a string, and don't want it to fail if there are zero occurrences, this code is what you need. Here's a demonstration:

 /* * Example */ const count = (str) => { const re = /[az]{3}/g return ((str || '').match(re) || []).length } const str1 = 'abc, def, ghi' const str2 = 'ABC, DEF, GHI' console.log(`'${str1}' has ${count(str1)} occurrences of pattern '/[az]{3}/g'`) console.log(`'${str2}' has ${count(str2)} occurrences of pattern '/[az]{3}/g'`)

Original Answer

The problem with your initial code is that you are missing the global identifier :

>>> 'hi there how are you'.match(/\s/g).length;
4

Without the g part of the regex it will only match the first occurrence and stop there.

Also note that your regex will count successive spaces twice:

>>> 'hi  there'.match(/\s/g).length;
2

If that is not desirable, you could do this:

>>> 'hi  there'.match(/\s+/g).length;
1

Answer 2

As mentioned in my earlier answer , you can use RegExp.exec() to iterate over all matches and count each occurrence; the advantage is limited to memory only, because on the whole it's about 20% slower than using String.match() .

var re = /\s/g,
count = 0;

while (re.exec(text) !== null) {
    ++count;
}

return count;

Answer 3

(('a a a').match(/b/g) || []).length; // 0
(('a a a').match(/a/g) || []).length; // 3

Based on https://stackoverflow.com/a/48195124/16777 but fixed to actually work in zero-results case.

Answer 4

('my string'.match(/\\s/g) || []).length;

Answer 5

This is certainly something that has a lot of traps. I was working with Paolo Bergantino's answer, and realising that even that has some limitations. I found working with string representations of dates a good place to quickly find some of the main problems. Start with an input string like this: '12-2-2019 5:1:48.670'

and set up Paolo's function like this:

function count(re, str) {
    if (typeof re !== "string") {
        return 0;
    }
    re = (re === '.') ? ('\\' + re) : re;
    var cre = new RegExp(re, 'g');
    return ((str || '').match(cre) || []).length;
}

I wanted the regular expression to be passed in, so that the function is more reusable, secondly, I wanted the parameter to be a string, so that the client doesn't have to make the regex, but simply match on the string, like a standard string utility class method.

Now, here you can see that I'm dealing with issues with the input. With the following:

if (typeof re !== "string") {
    return 0;
}

I am ensuring that the input isn't anything like the literal 0 , false , undefined , or null , none of which are strings. Since these literals are not in the input string, there should be no matches, but it should match '0' , which is a string.

With the following:

re = (re === '.') ? ('\\' + re) : re;

I am dealing with the fact that the RegExp constructor will (I think, wrongly) interpret the string '.' as the all character matcher \\.\\

Finally, because I am using the RegExp constructor, I need to give it the global 'g' flag so that it counts all matches, not just the first one, similar to the suggestions in other posts.

I realise that this is an extremely late answer, but it might be helpful to someone stumbling along here. BTW here's the TypeScript version:

function count(re: string, str: string): number {
    if (typeof re !== 'string') {
        return 0;
    }
    re = (re === '.') ? ('\\' + re) : re;
    const cre = new RegExp(re, 'g');    
    return ((str || '').match(cre) || []).length;
}

Answer 6

Using modern syntax avoids the need to create a dummy array to count length 0

const countMatches = (exp, str) => str.match(exp)?.length ?? 0;

Must pass exp as RegExp and str as String .

Answer 7

how about like this

function isint(str){
    if(str.match(/\d/g).length==str.length){
        return true;
    }
    else {
         return false
    }
}