[R] using regular expressions to retrieve a digit-digit-dot structure from a string

[Wacek Kusnierczyk]

Gabor Grothendieck wrote:
> On Tue, Jun 9, 2009 at 3:04 AM, Wacek
> Kusnierczyk<Waclaw.Marcin.Kusnierczyk at idi.ntnu.no> wrote:
>   
>> Gabor Grothendieck wrote:
>>     
>>> On Mon, Jun 8, 2009 at 7:18 PM, Wacek
>>> Kusnierczyk<Waclaw.Marcin.Kusnierczyk at idi.ntnu.no> wrote:
>>>
>>>       
>>>> Gabor Grothendieck wrote:
>>>>
>>>>         
>>>>> Try this.  See ?regex for more.
>>>>>
>>>>>
>>>>>
>>>>>           
>>>>>> x <- 'This happened in the 21. century." (the dot behind 21 is'
>>>>>> regexpr("(?![0-9]+)[.]", x, perl = TRUE)
>>>>>>
>>>>>>
>>>>>>             
>>>>> [1] 24
>>>>> attr(,"match.length")
>>>>> [1] 1
>>>>>
>>>>>
>>>>>           
>>>> yes, but
>>>>
>>>>    gregexpr('(?![0-9]+)[.]', 'a. 1. a1.', perl=TRUE)
>>>>    # 2 5 9
>>>>
>>>>         
>>> Yes, it should be:
>>>
>>>
>>>       
>>>> gregexpr('(?<=[0-9])[.]', 'a. 1. a1.', perl=TRU
>>>>         
> E)
>   
>>> [[1]]
>>> [1] 5 9
>>> attr(,"match.length")
>>> [1] 1 1
>>>
>>> which displays the position of every dot that is preceded
>>> immediately by a digit.  Or just replace gregexpr with regexpr
>>> if its intended that it match only one.
>>>
>>>       
>> i guess what was needed was something like
>>
>>    gregexpr('(?<=\\b[0-9]+)[.]', 'a. 1. a1.', perl=TRUE)
>>    # 5
>>
>> which won't work, however, because pcre does not support variable-width
>> lookbehinds.
>>     
>
> No, what I wrote was what I intended.   I don't think we are
> discussing the answer
> at this point but just the interpretation of what was intended.  


which amounts to discussing whether the answer is appropriate ;)

> You
> are including
> the word boundary in the question and I am not.  

indeed, and i think this was essential.  but irrespectively of whether
it really was or not, this sort of problem shows the insufficiency of a
lookbehind, and illustrates the use of the \K operator, so it will
hopefully be easier for the op and others to design the right pattern in
similar future cases.

vQ