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

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?

3 Answers

Try

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

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

Another solution

grep(pattern="\\d", x=vec) 

man 7 regex 

Within a bracket expression, the name of a character class enclosed in "[:" and ":]" stands for the list of all characters belonging to that class. Standard character class names are:

 alnum digit punct alpha graph space blank lower upper cntrl print xdigit 

Therefore a character class that is the sole member of a bracket expression will look like double-brackets, such as [[:digit:]]. As another example, consider that [[:alnum:]] is equivalent to [[:alpha:][:digit:]].