POSIX class What the class matches
[:alnum:] Alphabetic and numeric characters
[:alpha:] Alphabetic characters
[:blank:] Whitespace (space or tab characters)
[:cntrl:] Control characters
[:digit:] Digits
[:graph:] Graphic (non-blank) characters
[:lower:] Lowercase alphabetic characters
[:print:] Graphic or space characters
[:punct:] Punctuation characters
[:space:] Space, tab, newline, carriage return
[:upper:] Uppercase alphabetic characters
[:xdigit:] Hexadecimal digits (0-9, a-f, A-F)