[R] regular expression for nth character in a string

[David Winsemius]

On Apr 25, 2011, at 6:17 AM, Gonçalo Ferraz wrote:

> Hi, I have a string
>
> "InTrouble"
>
> and want to extract, say, the first two characters: "In"
> or the last three: "blee"
> or the 3rd, 4th, and 5th: "Trou"
>
> Is there an easy way of doing this quickly with regular expressions  
> in gsub, grep or similar?


Not greppish but seems to be the obvious approach:

 > substr("Trouble", 1,2)
[1] "Tr"
 > substr("Trouble", 3,5)
[1] "oub"

The greppish ways:

 > sub("(^..)(.*$)", "\\1", "Troubles")
[1] "Tr"

 > sub("(^..)(...)(.*$)", "\\2", "Troubles")
[1] "oub"
 > sub("(^..)(.{3})(.*$)", "\\2", "Troubles")
[1] "oub"

-- 

David Winsemius, MD
West Hartford, CT