XML in a Nutshell, 3rd Edition

Name

Proper Group/PE Nesting

Text from specification

Parameter entity replacement text must be properly nested with parenthesized groups. That is to say, if either of the opening or closing parentheses in a choice, seq, or Mixed construct is contained in the replacement text for a parameter entity, both must be contained in the same replacement text.

For interoperability, if a parameter entity reference appears in a choice, seq, or Mixed construct, its replacement text should contain at least one non-blank character, and neither the first nor last non-blank character of the replacement text should be a connector (| or ,).

Explanation

This constraint restricts the way parameter entities can be used to construct element declarations. It is similar to the Text from specification constraint in that parameter entities may not be used to complete or open new parenthesized expressions. It prevents the XML author from hiding significant syntax elements inside parameter entities.