jquery - How to insert new line after N char count in javascript? -

i have string may have new line '\n' char in it. want insert new line '\n' after every 4 (or n) chars in string.

for example:

1) input: "i 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' present in string

3) input: 12345\n67890

output: 1234\n5\n6789\n0

4) input: "1234\n56\n78901"

output: "1234\n56\n7890\n1"

so far have created function inserts '\n' after every 4 chars but not consider '\n' if present in 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 function on every keypress , pass original string , output , use further. hope understand meant here. should preserve inserted new lines.

let me know can explain further. more examples.

here best guess being asked :

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

some test strings , results :

 "i 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 of whom regular expressions mysterious here breakdown:

   ---------------------------- (?!     check following character(s) not                      |  -------------------------   $|\n  beginning of string or newline character                       |  |   --------------------- )                    |  |  | -------------------- (       start of capture group 1            |  |  ||  ------------------   [^\n] single character other newline               |  |  ||  |   --------------   {4}   previous element repeated 4 times            |  |  ||  |   |  -----------   (?!   check following character(s) not      |  |  ||  |   |  | ---------     \n  newline        |  |  ||  |   |  | | -------   )         |  |  ||  |   |  | | |------ )       end of capture group 1      |  |  ||  |   |  | | || ---- /g      in replace causes matches processed    |  |  ||  |   |  | | || |  /(?!$|\n)([^\n]{4}(?!\n))/g