For example, before the POSIX standard, to match alphanumeric characters, you would have had to write /[A-Za-z0-9]/. If your character set had other alphabetic characters in it, this would not match them, and if your character set collated differently from ASCII, this might not even match the ASCII alphanumeric char‐acters. With the POSIX character classes, you can write /[[:alnum:]]/, and this matches the alphabetic and numeric charac‐ters in your character set, no matter what it is.