regex - R regular expressions: unexpected behavior of "[:digit:]"

Question

I'd like to extract elements beginning with digits from a character vector but there's something about POSIX regular expression syntax that I don't understand.

I would think that

vec <- c("012 foo", "305 bar", "other", "notIt 7")
grep(pattern="[:digit:]", x=vec)

would return 1 2 4 since they are the four elements that have digits somewhere in them. But in fact it returns 3 4.

Likewise grep(pattern="^0", x=vec) returns 1 as I would expect because element 1 starts with a zero. However grep(pattern="^[:digit:]", x=vec) returns integer(0) whereas I would expect it to return 1 2 since those are the elements that start with digits.

How am I misunderstanding the syntax?

Answer 1

Try

grep(pattern="[[:digit:]]", x=vec)

instead as the 'meta-patterns' between colons usually require double brackets.