POSIX

[:upper:]

Upper case letters

[:lower:]

Lower case letters

[:alpha:]

All letters

[:alnum:]

Digits and letters

[:digit:]

Digits

[:xdigit:]

Hexadecimal digits

[:punct:]

Punctuation

[:blank:]

Space and tab

[:space:]

Blank characters

[:cntrl:]

Control characters

[:graph:]

Printed characters

[:print:]

Printed characters and spaces

[:word:]

Digits, letters and underscore