Extract numbers and operators from a given string

I have a vector like this:

 vec <- c("a + 17", "äÜ - 20*3")

There are different letters, numbers and operators. I want the get rid of the letters. Or, the other way around, to keep only the numbers and operators. Here is the result I am looking for:

 c("17", "-20*3")

I tried gsub("[:alpha:]", "", vec) but it doesn’t work and I don’t get why because [:alpha:] should remove any letters and then I should end up with the vector I am looking for. But this is not the case.

I found a similar question but it didn’t help me either.

>Solution :

Here is a different approach using gsub:

x <- gsub("[^0-9+-/*]", "", vec)

c(as.numeric(x)[1], x[2])

output:

[1] "17"    "-20*3"
Warning message:
NAs introduced by coercion