# String#split and groups in the field separator RE

**URL:** <https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896>\
**Category:** Ruby\
**Created:** [November 1, 2007, 5:37am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896 "2007-11-01T05:37:26Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 5:37am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/1 "2007-11-01T05:37:26Z")

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Is this expected behaviour? I haven’t seen anything related to this  
mentioned in the API docs…

irb(main):057:0\> s = ‘a:🅱::c::::d’  
=\> “a:🅱::c::::d”  
irb(main):058:0\> s.split(/:/)  
=\> [“a”, “”, “b”, “”, “”, “c”, “”, “”, “”, “d”] =\> OK  
irb(main):059:0\> s.split(/:+/)  
=\> [“a”, “b”, “c”, “d”] =\> OK  
irb(main):060:0\> s.split(/(:)+/)  
=\> [“a”, “:”, “b”, “:”, “c”, “:”, “d”] =\> ?  
irb(main):061:0\> s.split(/((:)+)/)  
=\> [“a”, “::”, “:”, “b”, “:::”, “:”, “c”, “::::”, “:”, “d”] =\> ???  
irb(main):062:0\> s.split(/(:+)/)  
=\> [“a”, “::”, “b”, “:::”, “c”, “::::”, “d”] =\> ???

mortee

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 6:20am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/2 "2007-11-01T06:20:32Z")

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mortee wrote:

> Is this expected behaviour? I haven’t seen anything related to this  
> mentioned in the API docs…
> 
> irb(main):057:0\> s = ‘a:🅱::c::::d’  
> =\> “a:🅱::c::::d”  
> irb(main):058:0\> s.split(/:/)  
> =\> [“a”, “”, “b”, “”, “”, “c”, “”, “”, “”, “d”] =\> OK  
> irb(main):059:0\> s.split(/:+/)  
> =\> [“a”, “b”, “c”, “d”] =\> OK  
> irb(main):060:0\> s.split(/(:)+/)  
> =\> [“a”, “:”, “b”, “:”, “c”, “:”, “d”] =\> ?  
> irb(main):061:0\> s.split(/((:)+)/)  
> =\> [“a”, “::”, “:”, “b”, “:::”, “:”, “c”, “::::”, “:”, “d”] =\> ???  
> irb(main):062:0\> s.split(/(:+)/)  
> =\> [“a”, “::”, “b”, “:::”, “c”, “::::”, “d”] =\> ???

It was unexpected behavior for me when I ran into it using python’s  
regex split() function a few months ago. Since it works the same way in  
both languages, I would guess it might be a universal regex trait.

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 6:43am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/3 "2007-11-01T06:43:21Z")

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mortee wrote:

> Is this expected behaviour? I haven’t seen anything related to this  
> mentioned in the API docs…
> 
> irb(main):057:0\> s = ‘a:🅱::c::::d’  
> =\> “a:🅱::c::::d”  
> irb(main):058:0\> s.split(/:/)  
> =\> [“a”, “”, “b”, “”, “”, “c”, “”, “”, “”, “d”] =\> OK  
> irb(main):059:0\> s.split(/:+/)  
> =\> [“a”, “b”, “c”, “d”] =\> OK  
> irb(main):060:0\> s.split(/(:)+/)  
> =\> [“a”, “:”, “b”, “:”, “c”, “:”, “d”] =\> ?  
> irb(main):061:0\> s.split(/((:)+)/)  
> =\> [“a”, “::”, “:”, “b”, “:::”, “:”, “c”, “::::”, “:”, “d”] =\> ???  
> irb(main):062:0\> s.split(/(:+)/)  
> =\> [“a”, “::”, “b”, “:::”, “c”, “::::”, “d”] =\> ???

I guess I should mention that the rule I jotted down in the margin of my  
book is: if the split() pattern has parenthesized sub groupings, the  
result array will include the match for each subgroup–but not the whole  
match.

Applying that rule to your examples:

> irb(main):060:0\> s.split(/(:)+/)  
> =\> [“a”, “:”, “b”, “:”, “c”, “:”, “d”] =\> ?

The subgroup (🙂 matches a single colon, so those matches are included  
in the results,

> irb(main):061:0\> s.split(/((:)+)/)  
> =\> [“a”, “::”, “:”, “b”, “:::”, “:”, “c”, “::::”, “:”, “d”] =\> ???

The subgroup (🙂 matches one colon and those results are included. The  
subgroup ((:)+) matches two, three, and four colons as it traverses the  
strings and those results are included. Because groups are numbered by  
their left most parentheses, the outer grouping comes first in the list.

> irb(main):062:0\> s.split(/(:+)/)  
> =\> [“a”, “::”, “b”, “:::”, “c”, “::::”, “d”] =\> ???

The subgroup (:+) matches two, three, and four colons as it traverses  
the list, and those matches are included in the results.

And, here is an example of my own that shows that the whole match is not  
included in the results–only the parenthesized sub groupings are  
included:

str = 'a\_::_b_:::_c_::::_d’  
pattern = /_(:+)\_/

results = str.split(pattern)  
p results

–output:–  
[“a”, “::”, “b”, “:::”, “c”, “::::”, “d”]

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 6:39am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/4 "2007-11-01T06:39:20Z")

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> Is this expected behaviour? I haven’t seen anything related to this  
> mentioned in the API docs…
> 
> irb(main):060:0\> s.split(/(:)+/)  
> =\> [“a”, “:”, “b”, “:”, “c”, “:”, “d”]

Yes, any capture groups in the regex will be included in the split  
array. If you want to use groups without capturing it into the split  
array, use a non-capturing group - ie, /(?=,)/ rather than /(,)/

It is curious that it’s not in the api doc… I must have learnt it from  
somewhere…

Dan.

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 6:58am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/5 "2007-11-01T06:58:38Z")

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mortee wrote:

> Is this expected behaviour? I haven’t seen anything related to this  
> mentioned in the API docs…

$ri String#split  
…  
…  
if _pattern_ is a +Regexp+, _str_ is divided where the pattern  
matches. Whenever the pattern matches a zero-length string, _str_  
is split into individual characters.  
…

pickaxe2, p. 619 adds a line to the end of that description:

…  
If pattern includes groups, these groups will be included in the  
returned values.

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 7:23am UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/6 "2007-11-01T07:23:09Z")

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Daniel S. wrote:

> Yes, any capture groups in the regex will be included in the split  
> array. If you want to use groups without capturing it into the split  
> array, use a non-capturing group - ie, /(?=,)/ rather than /(,)/
> 
> It is curious that it’s not in the api doc… I must have learnt it from  
> somewhere…

7stud – wrote:

> The subgroup (🙂 matches a single colon, so those matches are included  
> in the results,  
> […]

Thanks, that clarifies it, and the results make sense based on the rule.  
However, I find it quite confusing to have parts of what I intend to be  
part of the “separator” among the list of results. To say the least.

mortee

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**Author:** ![mortee](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/50afbb/32.png) [@mortee](https://www.ruby-forum.com/u/mortee)\
**Post date:** [November 1, 2007, 4:34pm UTC](https://www.ruby-forum.com/t/string-split-and-groups-in-the-field-separator-re/118896/7 "2007-11-01T16:34:28Z")

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Daniel S. wrote:

> > Is this expected behaviour? I haven’t seen anything related to this  
> > mentioned in the API docs…
> > 
> > irb(main):060:0\> s.split(/(:)+/)  
> > =\> [“a”, “:”, “b”, “:”, “c”, “:”, “d”]
> 
> Yes, any capture groups in the regex will be included in the split  
> array. If you want to use groups without capturing it into the split  
> array, use a non-capturing group - ie, /(?=,)/ rather than /(,)/

/(?=,)/ is a lookahead match. I’m sure you really meant /(?:,)/