Splitting a string into fixed length parts

Break a string up into substrings all of a known length

The trick is to use a look-behind with the regex \G, which means "end of previous match":

String[] parts = str.split("(?<=\\G.{8})");

The regex matches 8 characters after the end of the last match. Since in this case the match is zero-width, we could more simply say "8 characters after the last match".

Conveniently, \G is initialized to start of input, so it works for the first part of the input too.

Break a string up into substrings all of variable length

Same as the known length example, but insert the length into regex:

int length = 5;
String[] parts = str.split("(?<=\\G.{" + length + "})");

Remarks

The goal here is to not lose content, so the regex must not consume (match) any input. Rather it must match between the last character of the previous target input and the first character of the next target input. eg for 8-character substrings, we need to break the input up (ie match) at the places marked below:

a b c d e f g h i j k l m n o p q r s t u v w x y z
               ^               ^               ^

Ignore the spaces in the input which were required to show between character positions.