Guide on transforming a specified string into a modified regex string using JavaScript

Here are some example strings:

 2222 
 333333 
 12345 
 111
 123456789
 12345678

The expected result is:

2@222
333@333
12@345
111
123@456@789
12@345@678

In other words, the character '@' should be inserted at the 4th, 8th, 12th, etc. positions counting from the end of the string.

I think this can be achieved using replace and other methods available in JavaScript.

To validate the output string, a regular expression has been created:

^(\d{1,3})(\.\d{3})*?$

Answer №1

If you're looking to extract and group specific digits in a string, consider using the regular expression below:

/(\d)(\d{3})$/

This regex will identify and group the first digit as \d and the last three digits as \d{3}, creating separate groups for each set of digits. By referencing these matched groups with $1 and $2 in your replacement string, you can manipulate the output effectively.

Take a look at the example code snippet provided:

const transform = str => str.replace(/(\d)(\d{3})$/, '$1@$2');

console.log(transform("2222")); // 2@222
console.log(transform("333333")); // 333@333
console.log(transform("12345")); // 12@345
console.log(transform("111")); // 111

For longer strings with a size of N, an alternate approach would be to utilize methods like .match() and reverse the string before grouping the digits. Here's an example showcasing this technique:

const reverse = str => Array.from(str).reverse().join('');
const transform = str => {
  return reverse(reverse(str).match(/(\d{1,3})/g).join('@'));
}

console.log(transform("2222")); // 2@222
console.log(transform("333333")); // 333@333
console.log(transform("12345")); // 12@345
console.log(transform("111")); // 111
console.log(transform("123456789")); // 123@456@789
console.log(transform("12345678")); // 12@345@678

Answer №2

var numbers = [
  '111',
  '2222',
  '333333',
  '12345',
  '123456789',
  '1234567890123456'
];
console.log(numbers.map(function (number) {
  return number.replace(/(?=(?:\B\d{3})+$)/g, '@');
}));

Answer №3

To capture all the numbers, you can apply a pattern where an @ is added after every third digit from the right using a positive lookahead.

(?=(?:\B\d{3})+$)
  • (?= This denotes a Positive lookahead, checking what comes next
    • (?:\B\d{3})+ Repeats 1+ times and captures not a word boundary followed by 3 digits
  • $ Asserts the end of the string
  • ) Closes the lookahead

View Regex demo

const regex = /^\d+$/;
["2222",
  "333333",
  "12345",
  "111",
  "123456789",
  "12345678"
].forEach(s => console.log(
  s.replace(/(?=(?:\B\d{3})+$)/g, "@")
));

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