[R] test if elements of a character vector contain letters

[Marc Schwartz]

On Aug 7, 2012, at 3:02 PM, Liviu Andronic <landronimirc at gmail.com> wrote:

> On Mon, Aug 6, 2012 at 7:35 PM, Marc Schwartz <marc_schwartz at me.com> wrote:
>> is.letter <- function(x) grepl("[[:alpha:]]", x)
>> is.number <- function(x) grepl("[[:digit:]]", x)
>> 
> 
> Another follow-up. To test for (non-)alphanumeric one would do the following:
>> x <- c(letters, 1:26, '+', '-', '%^&')
>> x[1:10] <- paste(x[1:10], 1:10, sep='')
>> x
> [1] "a1"  "b2"  "c3"  "d4"  "e5"  "f6"  "g7"  "h8"  "i9"  "j10" "k"
> "l"   "m"   "n"
> [15] "o"   "p"   "q"   "r"   "s"   "t"   "u"   "v"   "w"   "x"   "y"
> "z"   "1"   "2"
> [29] "3"   "4"   "5"   "6"   "7"   "8"   "9"   "10"  "11"  "12"  "13"
> "14"  "15"  "16"
> [43] "17"  "18"  "19"  "20"  "21"  "22"  "23"  "24"  "25"  "26"  "+"
> "-"   "%^&"
>> xb <- grepl("[[:alnum:]]",x)  ##test for alphanumeric chars
>> x[xb]
> [1] "a1"  "b2"  "c3"  "d4"  "e5"  "f6"  "g7"  "h8"  "i9"  "j10" "k"
> "l"   "m"   "n"
> [15] "o"   "p"   "q"   "r"   "s"   "t"   "u"   "v"   "w"   "x"   "y"
> "z"   "1"   "2"
> [29] "3"   "4"   "5"   "6"   "7"   "8"   "9"   "10"  "11"  "12"  "13"
> "14"  "15"  "16"
> [43] "17"  "18"  "19"  "20"  "21"  "22"  "23"  "24"  "25"  "26"
>> xb <- grepl("[[:punct:]]",x)  ##test for non-alphanumeric chars
>> x[xb]
> [1] "+"   "-"   "%^&"


That will get you values where punctuation characters are used, but there may be other non-alphanumeric characters in the vector. There may be ASCII control codes, tabs, newlines, CR, LF, spaces, etc. which would not be found by using [:punct:].

For example:

> grepl("[[:punct:]]", " ")
[1] FALSE


If you want to explicitly look for non-alphanumeric characters, you would be better off using a negation of [:alnum:] such as:

grepl("[^[:alnum:]]", x)

or

!grepl("[[:alnum:]]", x)


Regards,

Marc



> 
> More regex rules are available on the Wiki [1]. Regards
> Liviu
> 
> [1] http://en.wikipedia.org/wiki/Regular_expression
> 
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