String Expansion

String expansion is a feature that allows strings to dynamically define their value at run time. For historical reasons, these string expansions are called "xlats".

String expansion is performed via the following syntax:

%{…}

Where the %{ signals the start of a dynamic expansion, and } signals the end of the dynamic expansion. The contents of the expansion can be many things:

Table 1. String Expansions
Keyword	Description
attributes	Expand the value of a named attribute.
single character	Single character expansions.
modules	Pass a string to a module such as `sql`.
condition	Conditionally expand a string.
built-in expansions	Such as string length, tolower, etc…

This feature is used to create policies which refer to concepts rather than to specific values. For example, a policy can be created that refers to the User-Name in a request, via:

%{User-Name}

This string expansion is done only for double-quoted strings and for the back-tick operator.

Caveats

Unlike other languages, there is no way to define new variables. All of the string expansions must refer to attributes that already exist, or to modules that will return a string value.

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.

Example

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