How to read BNF diagrams

In BNF, the syntax is written out in a line. Square brackets, [ and ], braces, { and }, and the vertical bar symbol, |, are used to describe the syntax. Collectively, these symbols are known as controls.

Blanks are used to separate the syntax elements from the square brackets, braces and vertical bars. Use your screen reader's "speak next word" (usually Control + RightArrow key) and "speak prior word" (usually Control + LeftArrow key) to move between successive BNF controls and syntax elements.

Characters that you need to include when you are constructing your command (for example, left and right brackets ( and ), and separators such as a comma) are located at the point at which you need to type them in. For example, a left bracket can be read out before a list of options. In some programming languages, such as C, C++, and Java, square brackets, braces and the vertical bar symbol are used within the language. For ease of identification, where the square bracket, brace or vertical bar symbol appears in the syntax diagram as a part of the programming language, it is preceded by the escape character, which is a \ (backslash). For example, if you hear \{, you know that you should actually type in the { at that point when you are constructing your command, and that the { is not a part of the structure of the syntax diagram.

The BNF syntax works as follows:

• Braces { and } are used to indicate a group of syntax elements, in the same way as ( and ) are used in algebra. For example, AUTH( { USERID | SYSTEMID } ) shows that USERID and SYSTEMID are the group of options for the AUTH keyword. Note that this example syntax includes a ( and ) that you need to type in around the option you select. If you selected the USERID option, your command would be AUTH(USERID).
• Square brackets [ and ] mean a construct that is optional. The optional construct is started by [ and ended by ]. The square brackets [ and ], and the braces { and }, can both be used on the same group of syntax elements. For example, the presence of the square brackets in the syntax [ AUTH( { USERID | SYSTEMID } ) ] means that you could leave out the keyword AUTH and its options from the command you construct.
• The vertical bar | is an OR sign and means an alternative. All constructs to the right of the | are an alternative to the construct to the left of the |. The scope of the | is delimited by either the end of the group or construct, shown by ] or }, or the end of the syntax. The vertical bar applies to all the items in the delimited group, not just those items immediately adjacent to the bar. For example, { A | B C } means "either A, or B and C". If you wanted to indicate "either A and C, or B and C", the correct syntax would be { A | B } C.
• Exclamation marks ! placed around a value indicate the default value in a set of alternatives. For example, AUTH( { !USERID! | SYSTEMID } ) means that USERID is the default option for the AUTH keyword. Take careful note of the position of any square brackets, [ and ], around a construct containing a default. If there are square brackets around the whole construct but not around the set of alternatives, for example [ AUTH( { !USERID! | SYSTEMID } ) ], this means that a default applies for the whole construct. If you omit the AUTH keyword from your finished command, the default setting AUTH(USERID) will be included automatically. However, if there are square brackets around the whole construct and also around the set of alternatives, for example [ AUTH [ ( { !USERID! | SYSTEMID } ) ] ], this means that a default applies only for the set of alternatives. In this case, if you omit the AUTH keyword from your finished command, no default setting will be included. If you include the AUTH keyword but do not state your choice of option, the default option USERID will be applied.
• An ellipse ... following a construct means that the syntax elements given in that construct can optionally be repeated. For example, { HOST | STATE | TYPE } ... means that you can include more than one of the keywords HOST, STATE, and TYPE. Note that if the construct only includes one syntax element, you can repeat that same syntax element more than once. However, if the construct includes more than one syntax element, you can use more than one of the elements, but you cannot use each element more than once. In the example above, you could write HOST STATE, but you could not write HOST HOST.
• A syntax element that starts and ends with a % (percentage symbol) means that the element is the name of a syntax fragment rather than a literal. It indicates a reference that is defined elsewhere. For example, %OP1% means that at this point, you should refer to the separate syntax fragment OP1. OP1, in the syntax from which this example was taken, gave a list of further options.