Dynamic Expansion

Dynamic expansion is a feature that allows values to be dynamically expanded at run time. For historical reasons, these string expansions are called "xlats".

Dynamic expansion is performed via the following syntax:

%{…}

Where the %{ signals the start of a dynamic expansion, and } signals the end of the dynamic expansion. Dynamic expansions are most commonly used in double-quoted strings, and expressions / conditions.

The contents of an expansion can be many things, such as attribute references, function calls, etc. The table below gives more examples of expansions.

Table 1. Types of Expansions
Keyword	Description
attributes	Expand the value of a named attribute.
single character	Single character expansions.
functions	Function call syntax
condition	Conditionally expand a string.
built-in expansions	Functions as string length, tolower, etc…

Expansions are used inside of double-quoted strings and for the back-tick quoted strings

Character Escaping

Some characters need to be escaped within a dynamically expanded string %{…}. The % character is used for variable expansion, so a literal % character can be created by using %%.

Other than within a dynamically expanded string, very little character escaping is needed. The rules of the enclosing string context determine whether or not a space or " character needs to be escaped. See the ref:type/string/double.adoc[double-quoted strings] and back-tick quoted strings pages for more information.

Example

&Reply-Message := "%{User-Name} with a literal %%"