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

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.