How to escape a question mark in R?

Question

I am trying to grep a vector of strings and some of them contain question marks.

I am doing:

grep('\?',vectorofStrings) and getting this error:

Error: '\?' is an unrecognized escape in character string starting "\?"

How can I determine the proper escaping procedure for '?'

Answer 1

You have to escape \ as well:

vectorOfStrings <- c("Where is Waldo?", "I don't know", "This is ? random ?")
grep("\\?", vectorOfStrings)
#-----
[1] 1 3

Answer 2

Use the \\ or fixed = TRUE argument as in:

vectorofStrings <-c("hello.", "where are you?",  "?")

grep('\\?',vectorofStrings)
grep('?',vectorofStrings, fixed=TRUE)

Answer 3

I'd guess \ is used in R as a normal string escape character, so to pass a literal \ to grep you might need \\?

Answer 4

I didn't have luck with backslash escaping, under windows grep. But I managed to make it work by the following:

grep [?]{3} *

That is, I enclosed the question mark in character class brackets ( [ and ] ), which made the special meaning inactive. The {3} part is not relevant to the question, I used it to find 3 consecutive question marks.