How to recognise and extract alpha numeric characters

Question

I want to extract alphanumeric characters from a partiular sentence in R. I have tried the following:

aa=grep("[:alnum:]","abc")

.This should return integer(0),but it returns 1,which should not be the case as "abc" is not an alphanumeric. What am I missing here? Essentially I am looking for a function,that only searches for characters that are combinations of both alphabets and numbers,example:"ABC-0112","PCS12SCH" Thanks in advance for your help.

Answer 1

[[:alnum:]] matches alphabets or digits. To match the string which contains the both then you should use,

x <- c("ABC", "ABc12", "--A-1", "abc--", "89=A")
grep("(.*[[:alpha:]].*[[:digit:]]|.*[[:digit:]].*[[:alpha:]])", x)
# [1] 2 3 5

or

which(grepl("[[:alpha:]]", x) & grepl("[[:digit:]]", x))
# [1] 2 3 5