Posix character class not recognized in a regex

[0liv1er]

Adding a character class inside a regular expression triggers a syntax error.

BEGIN{
        if($0 ~ /[[:digit:]]/) {
                print "has a digit"
        }
}
/[[:digit:]]/
/[[:alnum:]]/
/[[:alpha:]]/
/[[:blank:]]/
/[[:cntrl:]]/
/[[:digit:]]/
/[[:graph:]]/
/[[:lower:]]/
/[[:punct:]]/
/[[:print:]]/
/[[:space:]]/
/[[:upper:]]/
/[[:xdigit:]]/