Introducing UNIX and Linux

Character classes

`[:alnum:]`	letters and digits
`[:alpha:]`	letters
`[:blank:]`	usually Spaceand TAB
`[:cntrl:]`	all control characters
`[:digit:]`	digits
`[:graph:]`	printable characters excluding Space
`[:lower:]`	lower-case letters
`[:print:]`	all printable characters
`[:punct:]`	punctuation marks
`[:space:]`	whitespace characters
`[:upper:]`	upper-case letters
`[:xdigit:]`	hexadecimal digits (`0-9A-Fa-f`)