REGEX - Insert space after every 4 characters, and a line break after every 40 characters - regex

Question

I have a huge string (22000+ characters) of encoded text. The code is consisted of digits [0-9] and lower case letters [a-z]. I need a regular expression to insert a space after every 4 characters, and one to insert a line break [\n] after every fourty characters. Any ideas?

Answer 1

Many people would prefer to do this with a for loop and string concatenation, but I hate those substring calls. I am really against using regexes when they aren't the right tool for the job (parsing HTML), but I think it'd pretty easy to work with in this case.
JSFiddle Example
Let's say you have the string
var str = "aaaabbbbccccddddeeeeffffgggghhhhiiiijjjjkkkkllllmmmmnnnnoooo";
And you want to insert a space after every four characters, and a newline after 40 characters, you could use the following code
str.replace(/.{4}g/, function (value, index){
return value + (index % 40 == 36? '\n' : ' ');
});
Note that this wouldn't work if the newline(40) index wasn't a multiple of the space index(4)
I abstracted this in a project, here's a simple way to do it
/**
* Adds padding and newlines into a string without whitespace
* #param {str} str The str to be modified (any whitespace will be stripped)
* #param {int} spaceEvery number of characters before inserting a space
* #param {int} wrapeEvery number of spaces before using a newline instead
* return {string} The replaced string
*/
function addPadding(str, spaceEvery, wrapEvery) {
var regex = new RegExp(".{"+spaceEvery+"}", "g");
// Add space every {spaceEvery} chars, newline after {wrapEvery} spaces
return str.replace(/[\n\s]/g, '').replace(regex, function(value, index) {
// The index is the group that just finished
var newlineIndex = spaceEvery * (wrapEvery - 1);
return value + ((index % (spaceEvery * wrapEvery) === newlineIndex) ? '\n' : ' ');
});
}

Answer 2

Well, a regexp in itself doesn't insert a space, so I'll assume you have some command in whatever language you're using that inserts based on finding a regexp.
So, finding 4 characters and finding 40 characters: that's not pretty in general regular expressions (unless your particular implementation has nice ways to express numbers). For finding 4 characters, use
....
Because typical regexp finders use maximal munch, then from the end of one regexp, search forward and maximally munch again, that'll chunk your string into 4 character pieces. The ugly part is that in standard regular expressions, you'll have to use
........................................
to find chuncks of 40 characters, although I'll note that if you run your 4 character one first, you'll have to run
..................................................
or
.... .... .... .... .... .... .... .... .... ....
to account for the spaces you've already put in.
The period finds any characters, but given that you're only using [0-9|a-z], you could use that regexp in place of each period if you need to ensure nothing else slipped in, I was just avoiding making it even more gross.
As you may be noting, regexp have some limitations. Take a look at the Chomsky hierarchy to really get into their theoretical limitations.