Splitting a string into substrings of equal size

[Brian]

On Sat, Aug 15, 2009 at 4:06 PM, MRAB <python at mrabarnett.plus.com> wrote:

> ryles wrote:
>
>> On Aug 14, 8:22 pm, candide <cand... at free.invalid> wrote:
>>
>>> Suppose you need to split a string into substrings of a given size
>>> (except
>>> possibly the last substring). I make the hypothesis the first slice is at
>>> the
>>> end of the string.
>>> A typical example is provided by formatting a decimal string with
>>> thousands
>>> separator.
>>>
>>> What is the pythonic way to do this ?
>>>
>>> For my part, i reach to this rather complicated code:
>>>
>>> # ----------------------
>>>
>>> def comaSep(z,k=3, sep=','):
>>>    z=z[::-1]
>>>    x=[z[k*i:k*(i+1)][::-1] for i in range(1+(len(z)-1)/k)][::-1]
>>>    return sep.join(x)
>>>
>>> # Test
>>> for z in ["75096042068045", "509", "12024", "7", "2009"]:
>>>    print z+" --> ", comaSep(z)
>>>
>>> # ----------------------
>>>
>>> outputting :
>>>
>>> 75096042068045 -->  75,096,042,068,045
>>> 509 -->  509
>>> 12024 -->  12,024
>>> 7 -->  7
>>> 2009 -->  2,009
>>>
>>> Thanks
>>>
>>
>> py> s='1234567'
>> py> ','.join(_[::-1] for _ in re.findall('.{1,3}',s[::-1])[::-1])
>> '1,234,567'
>> py> # j/k ;)
>>
>
> If you're going to use re, then:
>
> >>> for z in ["75096042068045", "509", "12024", "7", "2009"]:
>        print re.sub(r"(?<=.)(?=(?:...)+$)", ",", z)
>
>
> 75,096,042,068,045
> 509
> 12,024
> 7
> 2,009
>

Can you please break down this regex?
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