Missing POSIX notation like [:digit:] and [:alpha:]

Look at the table at the middle of this page.

Everytime you write [a-zA-Z0-9] to select letters and digits, you can write [:alnum:] instead. Another example is when you want to select whitespace. You can write [ \t\r\n\v\f] for spaces, tabs, carriage return, newlines, vertical tabs and page breaks (all of which you would have to remember), or you write [:space:].

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