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

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Allen 2020-1-26

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评论： Allen 2020-1-27

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.

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Walter Roberson 2020-1-26

Is it permitted to use more than one call to regexp() or regexprep(), or does it need to be just a single call to one or the other?

Is the "execute" group of regexprep() to be permitted?

Allen 2020-1-27

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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Stephen23 2020-1-26

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编辑：Stephen23 2020-1-27

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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Walter Roberson 2020-1-27

I was thinking vaguely about recursion ;-)

Allen 2020-1-27

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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