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.

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Answer 1

Try

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

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

Answer 2

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:]].

Answer 3

Another solution

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