In all cases the value of string2 is defined as all characters, if any, after the <equals-sign>, up to a comment character ('#') or an unescaped <newline>. Any <blank> characters immediately after the <equals-sign> or immediately before the <plus-sign> shall be ignored.

In all cases the value of string1 is defined as all characters from the first non-<blank> character to the last non-<blank> character, inclusive, before the <tt>=</tt>, <tt>::=</tt>, <tt>?=</tt>, or <tt>+=</tt>. Portable applications shall ensure that a <blank> precedes the <tt>::=</tt>, <tt>?=</tt>, or <tt>+=</tt> in those forms to avoid any parsing ambiguity with implementations that permit <colon>, <question-mark>, or <plus-sign> in macro names as extensions. The value of string2 is defined as all characters from the first non-<blank> character, if any, after the <equals-sign>, up to but not including a comment character ('#') or an unescaped <newline>.

Applications shall select macro names from the set of characters consisting solely of characters from the portable character set (see [xref to XBD 6.1). A macro name shall not contain an <equals-sign>. Implementations may allow other characters in macro names as extensions.

Portable applications shall select macro names from the set of characters consisting solely of characters from the portable filename character set. Implementations may allow other characters in macro names as extensions; however, a macro name shall not contain an <equals-sign>, <blank>, or control character.