R help - May 2008 - Regular Expressions

Hi R,

 

Again struck with regular expressions...

 

Suppose,

 

S=c("World_is_beautiful",
"one_two_three_four","My_book")

 

I need to extract the last but one element of the strings. So, my output should
look like:

Ans=c("is","three","My")

 

gsub() can do this...but wondering how do I give the regular expression....

 

 

 

Shubha Karanth | Amba Research

Ph +91 80 3980 8031 | Mob +91 94 4886 4510 

Bangalore * Colombo * London * New York * San José * Singapore *
www.ambaresearch.com

 

This e-mail may contain confidential and/or privileged i...{{dropped:13}}

try this:

S <- c("World_is_beautiful",
"one_two_three_four","My_book")

sapply(strsplit(S, "_"), tail, n = 2)[1, ]
# or
sapply(strsplit(S, "_"), function(x) x[length(x) - 1])


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Shubha Vishwanath Karanth" <shubhak at ambaresearch.com>
To: <r-help at stat.math.ethz.ch>
Sent: Tuesday, May 13, 2008 11:02 AM
Subject: [R] Regular Expressions


Hi R,



Again struck with regular expressions...



Suppose,



S=c("World_is_beautiful",
"one_two_three_four","My_book")



I need to extract the last but one element of the strings. So, my 
output should look like:

Ans=c("is","three","My")



gsub() can do this...but wondering how do I give the regular 
expression....







Shubha Karanth | Amba Research

Ph +91 80 3980 8031 | Mob +91 94 4886 4510

Bangalore * Colombo * London * New York * San Jos? * Singapore * 
www.ambaresearch.com



This e-mail may contain confidential and/or privileged 
i...{{dropped:13}}




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Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

> S=c("World_is_beautiful",
"one_two_three_four","My_book")
> I need to extract the last but one element of the strings. So, my 
> output should look like:
 
> Ans=c("is","three","My")
> gsub() can do this...but wondering how do I give the regular 
expression....

sapply(strsplit(S, "_"), function(x) x[length(x)-1])

You could use regular expressions, but I think it would only be 
complicating things.

Regards,
Richie.

Mathematical Sciences Unit
HSL


------------------------------------------------------------------------
ATTENTION:

This message contains privileged and confidential inform...{{dropped:20}}

On Tue, May 13, 2008 at 5:02 AM, Shubha Vishwanath Karanth
<shubhak at ambaresearch.com> wrote:
> Suppose,
>
> S=c("World_is_beautiful",
"one_two_three_four","My_book")
>
> I need to extract the last but one element of the strings. So, my output
should look like:
>
> Ans=c("is","three","My")
>
> gsub() can do this...but wondering how do I give the regular expression....
>
As others have mentioned strsplit is probably easier in this case but it can
be done with a regular expression as shown below where [^_]+ matches a
any string of characters not containing _ :
> re <- "^([^_]+_)*([^_]+)_([^_]+)$"
> gsub(re, "\\2", S)
[1] "is"    "three" "My"

The strapply function in the gsubfn package can also be used.
out below has the same value as strsplit(S, "_"):

library(gsubfn)
out <- strapply(S, "[^_]+")
sapply(out, function(x) tail(x, 2)[1])