How to split a string of digits into groups of three from right-to-left using only regular expressions?

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While I have been able to accomplish this task using a varitey of mixed functions such as regexp() and fliplr(), I am ultimately stumped when trying to resort to only regexp() and/or regexprep(). For example, I am trying to convert the a string similar to the following:
str = '12345678';
such that the resulting output is:
{'12'} {'345'} {'678'}
I am also trying to accomplish this task without the use of loops of any sort.
Allen on 27 Jan 2020
Single call was preferred, but multiple are acceptable. Was hoping to stay away from the execute groups since that technically uses other functions. However, since I have no experience with trying to generate code using them, I would not have minded seeing a few examples. ;-)

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Accepted Answer

Stephen23 on 26 Jan 2020
Edited: Stephen23 on 27 Jan 2020
With one regexp call (uses a lookaround assertion):
>> str = '12345678';
>> regexp(str, '(^\d{1,2}(?=(\d{3})*$)|\d{3})','match')
ans =
'12' '345' '678'
With two regexp calls (splits string into two tokens, may have empty cells in the output):
>> tkn = regexp(str,'^(\d{0,2})((\d{3})*)$','tokens','once');
>> out = [tkn(1),regexp(tkn{2},'\d{3}','match')]
out =
'12' '345' '678'
or with some extra functions to define the regular expression itself:
>> rgx = ['(\d{0,2})',repmat('(\d{3})',1,fix(numel(str)/3))];
>> regexp(str,rgx,'tokens','once')
ans =
'12' '345' '678'
Allen on 27 Jan 2020
The first two work perfectly under the limitations. I have been trying to solve that one for quite some time now, but had not been successful with implementing a lookaround assertion to capture more than the first token. Thanks to you both.

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