Retain only N instances of a particular character within a JavaScript string

Question

Retain only N instances of a particular character within a JavaScript string

Suppose I have a string like this: "a_b_c_d_restofthestring" and I want to keep only 2 underscores. For example,

 "a_b_cdrestofthestring"
 "abc_d_restofthestring"

Both of these are valid outputs.

This is my current implementation:

let str = "___sdaj___osad$%^&*";

document.getElementById('input').innerText = str;

let u = 0;
str = str.split("").reduce((output, c) => {
  if (c == "_") u++;
  return u < 2 || c != "_" ? output + c : output;
});

document.getElementById('output').innerText = str;

<div id="input"></div>
<div id="output"></div>

I'm wondering if there's a more efficient way to achieve this...

javascript string

Answer 1

Answer №1

Your code is running smoothly. However, here's a concise regular expression that removes all underscores except for the last two in the input string.

let input = "___sdaj___osad$%^&*";
let output = input.replace(/_(?=(.*_){2})/g, '');

console.log("input: " + input);
console.log("output: " + output);

This approach is not very versatile as you would need to tweak the regular expression whenever you want to replace a different character or allow more than 2 occurrences. Nevertheless, if you are comfortable with this limitation, this solution requires less code maintenance.

Update: Here is an improved version that is fully generic and should offer better performance:

let input = "___sdaj___osad$%^&*";

function replace(input, char = '_', max = 2, replaceWith = '') {
  let result = "";
  const len = input.length;
  
  for (let i = 0, u = 0; i < len; i++) {
    let c = input[i];
    result += (c === char && ++u > max) ? replaceWith : c;
  }
  
  return result;
}

console.log("input: ", input);
console.log("output: ", replace(input));

Check out this jsPerf analysis.

Answer 2

Your code is running smoothly. However, here's a concise regular expression that removes all underscores except for the last two in the input string.

let input = "___sdaj___osad$%^&*";
let output = input.replace(/_(?=(.*_){2})/g, '');

console.log("input: " + input);
console.log("output: " + output);

This approach is not very versatile as you would need to tweak the regular expression whenever you want to replace a different character or allow more than 2 occurrences. Nevertheless, if you are comfortable with this limitation, this solution requires less code maintenance.

Update: Here is an improved version that is fully generic and should offer better performance:

let input = "___sdaj___osad$%^&*";

function replace(input, char = '_', max = 2, replaceWith = '') {
  let result = "";
  const len = input.length;
  
  for (let i = 0, u = 0; i < len; i++) {
    let c = input[i];
    result += (c === char && ++u > max) ? replaceWith : c;
  }
  
  return result;
}

console.log("input: ", input);
console.log("output: ", replace(input));

Check out this jsPerf analysis.

Answer 3

Answer №2

To transform a string, you can utilize a regular expression that identifies an underscore along with a counter to keep track of the underscores that need to be retained while replacing the rest.

var text = "a_b_c_d_restofthestring",
    result = text.replace(/_/g, (char => count => char && count-- ? '_' : '')(2));
    
console.log(result);

Answer 4

To transform a string, you can utilize a regular expression that identifies an underscore along with a counter to keep track of the underscores that need to be retained while replacing the rest.

var text = "a_b_c_d_restofthestring",
    result = text.replace(/_/g, (char => count => char && count-- ? '_' : '')(2));
    
console.log(result);

Retain only N instances of a particular character within a JavaScript string

Answer №1

Answer №2

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