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The example above defines several terminals and two commands : `usercommand` and `passwordcommand`. The `ALPHA` terminal contains all letters in upper and lower case. In the `ALPHA` rule, `%x41` corresponds to ASCII character code 41 in hexadecimal, i.e. capital `A`. The `CR` and `LF` terminals correspond to the carriage return and linefeed control characters. The `CRLF` rule concatenates these two terminals to match the standard end of line termination. The `DIGIT` terminal contains all digits. The `SP` terminal corresponds to the white space characters. The `usercommand` is composed of two strings separated by white space. In the ABNF rules that define the messages used by Internet applications, the commands are case-insensitive. The rule `"user"` corresponds to all possible cases of the letters that compose the word between brackets, e.g. `user`, `uSeR`, `USER`, `usER`, ... A `username` contains at least one letter and up to 8 letters. User names are case-sensitive as they are not defined as a string between brackets. The `password` rule indicates that a password starts with a letter and can contain any number of letters or digits. The white space and the control characters cannot appear in a `password` defined by the above rule.
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