The simplest way to use a variable is to include it in an attribute value, prefixed with the percent character (%). (%% denotes the character itself.) If the character directly after the % is a digit or a minus sign (-), the end of the variable name is considered to be the first non-digit character. Otherwise, the end of the name is considered to be the first non-alphanumeric, non-underscore character. In:
the first variable name is abc and the second is 2. It is also possible to specify the end of the variable name explicitly by enclosing the name in curly brackets:
When evaluated, a compound string like this produces a string value that concatenates variable values and simple text fragments, or fails if any of the variables is undefined.