I have a string which may have new line '\n' char in it. Now I want to insert new line '\n' after every 4 (or N) chars in that string.

For example:

1) INPUT: "I am John Doe."

OUTPUT: "I am\nJohn\nDoe"

In above example inserting '\n' after 4 char including space

2) INPUT: "I\nam John Doe"

OUTPUT: "I\nam J\nohn \nDoe"

In above example inserting space after 4 chars after first '\n' already present in the string

3) INPUT: 12345\n67890

OUTPUT: 1234\n5\n6789\n0

4) INPUT: "1234\n56\n78901"

OUTPUT: "1234\n56\n7890\n1"

So far I have created a function which inserts '\n' after every 4 chars but it does not consider '\n' if it is already present in the original string.

function addNewlines(str) {
  if (str.length >= 4) {
    var result = '';
    while (str.length > 0) {
      result += str.substring(0, 4) + '\n';
      str = str.substring(4);
    }
    return result;
  }
  return str;
}

I call this function on every keypress and pass the original string and get output and use it further. I hope you understand what I meant to say here. It should preserve the previously inserted new lines.

Let me know I can explain further. With more examples.

I have a string which may have new line '\n' char in it. Now I want to insert new line '\n' after every 4 (or N) chars in that string.

For example:

1) INPUT: "I am John Doe."

OUTPUT: "I am\nJohn\nDoe"

In above example inserting '\n' after 4 char including space

2) INPUT: "I\nam John Doe"

OUTPUT: "I\nam J\nohn \nDoe"

In above example inserting space after 4 chars after first '\n' already present in the string

3) INPUT: 12345\n67890

OUTPUT: 1234\n5\n6789\n0

4) INPUT: "1234\n56\n78901"

OUTPUT: "1234\n56\n7890\n1"

So far I have created a function which inserts '\n' after every 4 chars but it does not consider '\n' if it is already present in the original string.

function addNewlines(str) {
  if (str.length >= 4) {
    var result = '';
    while (str.length > 0) {
      result += str.substring(0, 4) + '\n';
      str = str.substring(4);
    }
    return result;
  }
  return str;
}

I call this function on every keypress and pass the original string and get output and use it further. I hope you understand what I meant to say here. It should preserve the previously inserted new lines.

Let me know I can explain further. With more examples.

• javascript
• jquery
• algorithm

• Please confirm your second example, The first inserted \n is after 5 characters, why? More examples would help also. I also find the * around the \n characters confusing, make the input and output exactly the strings you need. – HBP Commented May 19, 2013 at 7:59
• I have removed the *. Please do not consider the quotes it is just to represent string. – ʞɹᴉʞ ǝʌɐp Commented May 19, 2013 at 8:22
• Your description says you want to add a newline after every four characters (or four characters after an existing newline), but your examples show only a single newline added to the string. Which is it, only one newline added in total, or a newline after every four characters? – Michael Geary Commented May 19, 2013 at 8:25
• Thanks, but your second example output is still confusing, should there not be a \n before Doe? You also state "inserting space after 4 characters" : under what conditions? – HBP Commented May 19, 2013 at 8:25
• 1 Shouldn't your first example have one more \n too? And what should this string be turned into: 'ABCD\nEFGH\nIJKL' - should it simply preserve the existing newlines, or add additional newlines? The reason we're all guessing is that you haven't written a plete specification. You need to explain what you want in every edge case and provide a prehensive list of inputs and outputs. – Michael Geary Commented May 19, 2013 at 8:33

5 Answers 5

Here is my best guess as to what is being asked for :

function addNewLines (str) { 
      return str.replace (/(?!$|\n)([^\n]{4}(?!\n))/g, '$1\n');
}

Some test strings and their results :

 "I am John Doe.",   -> "I am\n Joh\nn Do\ne."
 "I\nam John Doe",   -> "I\nam J\nohn \nDoe"
 "12345\n67890",     -> "1234\n5\n6789\n0"
 "1234\n56\n78901",  -> "1234\n56\n7890\n1"
 "ABCD\nEFGH\nIJKL", -> "ABCD\nEFGH\nIJKL\n"
 "1234",             -> "1234\n"
 "12341234"          -> "1234\n1234\n"

For those of you for whom regular expressions are mysterious here is a breakdown:

   ---------------------------- (?!     Check that following character(s) are not                  
   |  -------------------------   $|\n  Beginning of string or a newline character                   
   |  |   --------------------- )                
   |  |  | -------------------- (       Start of capture group 1        
   |  |  ||  ------------------   [^\n] Any single character other than newline           
   |  |  ||  |   --------------   {4}   Previous element repeated exactly 4 times        
   |  |  ||  |   |  -----------   (?!   Check that following character(s) are not  
   |  |  ||  |   |  | ---------     \n  a newline    
   |  |  ||  |   |  | | -------   )     
   |  |  ||  |   |  | | |------ )       End of capture group 1  
   |  |  ||  |   |  | | || ---- /g      in replace causes all matches to be processed
   |  |  ||  |   |  | | || |
 /(?!$|\n)([^\n]{4}(?!\n))/g

function parseInput(str, char, length){
    var split = str.split(char),
        regex = RegExp('(.{' + length + '})','g');

    split[split.length-1] = split[split.length - 1].replace(regex, '$1' + char);
    return split.join(char);
}

console.log(parseInput("I am John Doe.", "\n", 4)); 
// output = "I am\n Joh\nn Do\ne."

1. Split the string by "\n" str.split("\n"). You get an array of strings.
2. Do your additional parsing and manipulation checking the element length and putting the result in a new array results.
3. Join the strings using results.join("\n").

This will remove "\n" duplicates too if you avoid to append or prepend "\n" to results elements.

Here is my code:

function ngram_insert (n, ins, input)
{
    var output = "";
    var i = 0;

    while (i < strlen(input))
    {
        if (i > 0 && i % n == 0)
        {
            output += ins;
        }

        output += input[i];

        i++;
    }

    return output;
}

Test:

var output = ngram_insert (3, "\n", "This is a test.");

function f(n, ins, str) {

      if (str.length == 0)
            return "";

      var i = str.indexOf("\n"), len = str.length, newStr;
      if (i == -1) {
            i = 1;
            newStr = str[0];
      }
      else {
           newStr = str.substring(0, i + 1);
      }

      var k = 1;

      while (k + i < len) {
            newStr += str[k + i];

            if (k % n == 0) {
                  newStr += ins;
            }

            k++;
      }

      return newStr;
}

Calling f(4, "\n", "I\nam John Doe"); returns "I\nam J\nohn \nDoe"