The grammar of human language is rich with a variety of sentence structures, verb tenses, and all sorts of irregular constructs and exceptions to the rules. Nonetheless, you mastered most of it by the age of three. Computer language grammars typically are simple, regular, and have few exceptions. In fact, computer grammars use only four rules to define how elements of a language may be arranged: sequence, choice, grouping, and repetition.
Sequence rules define the exact order in which elements appear in a language. For instance, if a sequence grammar rule states that element A is followed by B and then by C, your document must provide elements A, B, and C in that exact order. A missing element (A and C, but no B, for example), an extra element (A, B, E, then C), or an element out of place (C, A, then B) violates the rule and does not match the grammar.
In many grammars, XML included, sequences are defined by simply listing the appropriate elements, in order and separated by commas. Accordingly, our example sequence in the DTD would appear simply as A, B, C.
Choice grammar rules provide flexibility by letting the DTD author choose one element from among a group of valid elements. For example, a choice rule might state that you may choose elements D, E, or F; any one of these three elements would satisfy the grammar. Like many other grammars, XML denotes choice rules by listing the appropriate choices separated by a vertical ...