POSIX character set cheat sheet
[:alnum:] Alphanumeric characters
[:alpha:] Alphabetical characters
[:blank:] Space and tab characters
[:cntrl:] Control characters
[:digit:] Numerical digits
[:graph:] Printable and visible (excluding space) characters
[:lower:] Lowercase characters
[:print:] Printable characters (including whitespace)
[:punct:] Punctuation characters
[:space:] Whitespace characters
[:upper:] Uppercase characters
[:xdigit:] Hexadecimal characters