POSIX character classes

[:alnum:]

Letters and numbers, as in [a-zA-Z0-9]

[:alpha:]

Letters, as in [a-zA-Z]

[:blank:]

Blanks

[:cntrl:]

Controls

[:digit:]

Numbers, as in [0-9]

[:graph:]

Printable characters (blanks excluded)

[:lower:]

Small letters, as in [a-z]

[:print:]

Characters which are not controls

[:punct:]

Punctuations

[:space:]

Spaces

[:upper:]

Capital letters, as in [A-Z]

[:xdigit:]

Characters used to express a hexadecimal number, as in [0-9a-fA-F]