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

MATLABregexpregexprepregular expressions

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.

Best Answer

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'
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