alpha represents a letter of the alphabet (either upper- or lowercase). This is equivalent to [A-Za-z].
digit represents a digit between 09 (equivalent to [0-9]).
alnum represents an alphanumeric character, just like [0-9A-Za-z].
blank represents "blank" characters, normally space and Tab.
cntrl represents "control" characters, such as DEL, INS, and so forth.
graph represents all the printable characters except the space.
lower represents lowercase letters of the alphabet only.
upper represents uppercase letters of the alphabet only.
print represents all printable characters.
punct represents punctuation characters such as "." or ",".
space is the whitespace.
xdigit represents hexadecimal digits.