[R] extract date

[Petr Pikal]

Dear Prof.Ripley

Thank you for your answer. After some tests and errors I finished 
with suitable extraction function which gives me substatnial 
increase in positive answers. 

Nevertheless I definitely need to gain more practice in regular 
expressions, but from the help page I can grasp only easy things. Is 
there any "Regular expressions for dummies" available?

Best regards
Petr Pikal


On 5 Apr 2005 at 10:23, Prof Brian Ripley wrote:

> On Tue, 5 Apr 2005, Petr Pikal wrote:
> 
> > Dear all,
> >
> > please, is there any possibility how to extract a date from data
> > which are like this:
> 
> Yes, if you delimit all the possibilities.
> 
> > ....
> > "Date: Sat, 21 Feb 04 10:25:43 GMT"
> > "Date: 13 Feb 2004 13:54:22 -0600"
> > "Date: Fri, 20 Feb 2004 17:00:48 +0000"
> > "Date: Fri, 14 Jun 2002 16:22:27 -0400"
> > "Date: Wed, 18 Feb 2004 08:53:56 -0500"
> > "Date: 20 Feb 2004 02:18:58 -0600"
> > "Date: Sun, 15 Feb 2004 16:01:19 +0800"
> > ....
> >
> > I used
> >
> > strptime(paste(substr(x,12,13), substr(x,15,17), substr(x,19,22),
> > sep="-"), format="%d-%b-%Y")
> >
> > which suits to lines 3:5 and 7 (such are the most common in my
> > dataset) but obviously does not work with other lines.
> 
> For those examples, in character vector 'dates' (without quotes):
> 
> > nd <- gsub("^[^0-9]*([0-9]+) ([A-Za-z]+) ([0-9]+).*",
>               "\\1 \\2 \\3", dates)
> > strptime(nd, "%d %b %y")
> [1] "2004-02-21" "2020-02-13" "2020-02-20" "2020-06-14" "2020-02-18"
> [6] "2020-02-20" "2020-02-15"
> 
> You should be able to amend the regexp for a wider range of forms, but
> your first line is ambiguous (2004 or 2021?) so there are limits.
> 
> > If there is no stightforward solution I can live with what I use now
> > but some automagical function like
> >
> > give.me.date.from.my.string.regardles.of.formating(x)
> > would be great.
> 
> It would be impossible: when Americans write 07/04/2004 they do not
> mean April 7th.
> 
> -- 
> Brian D. Ripley,                  ripley at stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self) 1 South
> Parks Road,                     +44 1865 272866 (PA) Oxford OX1 3TG,
> UK                Fax:  +44 1865 272595
> 
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Petr Pikal
petr.pikal at precheza.cz