How to Count Character and String Occurrences in JavaScript Strings, Including Length Validation?

Counting Character and String Occurrences in Strings with JavaScript

Strings in JavaScript are powerful data structures that can manipulate text and perform various operations. A frequent task is counting the number of occurrences of a character or a string pattern within a larger string. Let's explore how to achieve this in JavaScript while considering additional validation criteria.

Counting Character Occurrences

Suppose we have a string containing characters we want to count. To count the occurrences of a specific character, like a comma (','), we can use the following approach:

var mainStr = "str1,str2,str3,str4";
console.log(("str1,str2,str3,str4".match(/,/g) || []).length);

The code uses the match() method to find all occurrences of the ',' character in the mainStr string. The returned array is then converted into a truthy array using the logical OR operator (||) and finally counted for its length. This method will correctly count the number of occurrences of the specified character in the string.

Counting String Patterns

We can also count the number of times a particular string pattern occurs within another string. For instance, counting the number of individual strings separated by commas in the mainStr variable:

console.log(("str1,str2,str3,str4".match(new RegExp("str", "g")) || []).length);

Here, we use a regular expression to find all occurrences of the 'str' pattern within the string. Similarly, we convert the returned array into a truthy array and count its length. This approach effectively counts the number of string patterns matching the given criteria.

Validating String Lengths

In addition to counting occurrences, we may need to validate the lengths of individual strings within the larger string. We can extend our previous regular expression to check the length of each string:

var re = new RegExp("str{1,15}", "g");
var matches = ("str1,str2,str3,str4".match(re) || []).length;

if (matches === 4) {
  console.log("All strings are valid and shorter than 15 characters.");
} else {
  console.log("Invalid strings found. Some string is not within the 15-character limit.");
}

This regular expression matches strings that start with 'str' and have a length between 1 and 15 characters. If the number of matching strings is equal to the expected number (4 in this case), we can conclude that all strings meet the validation criteria. Otherwise, we know that some strings exceed the maximum allowed length.

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