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

User · Accepted Answer

tl dr  Generic Pattern Counter     THIS IS WHAT YOU NEED const count    str    gt      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    x000D   x000D     x000D      Example x000D      x000D   x000D  const count    str    gt    x000D    const re     a-z  3  g x000D    return   str        match re         length x000D    x000D   x000D  const str1    abc  def  ghi  x000D  const str2    ABC  DEF  GHI  x000D   x000D  console log     str1   has   count str1   occurrences of pattern    a-z  3  g    x000D  console log     str2   has   count str2   occurrences of pattern    a-z  3  g    x000D   x000D   x000D    Original Answer  The problem with your initial code is that you are missing the global identifier    gt  gt  gt   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    gt  gt  gt   hi  there  match   s g  length  2   If that is not desirable  you could do this    gt  gt  gt   hi  there  match   s  g  length  1

User · Answer

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

User · Answer

my string  match   s g         length

User · Answer

how about like this  function isint str       if str match   d g  length  str length           return true            else            return false

User · Answer

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

User · Answer

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

[javascript] Count number of matches of a regex in Javascript

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