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= 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:
.String Expansions
[options="header"]
|=====
| Keyword | Description
| xref:xlat/attribute.adoc[attributes] | Expand the value of a named attribute.
| xref:xlat/character.adoc[single character] | Single character expansions.
| xref:xlat/module.adoc[modules] | Pass a string to a module such as `sql`.
| xref:xlat/alternation.adoc[condition] | Conditionally expand a string.
| xref:xlat/builtin.adoc[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 %%`
// Copyright (C) 2020 Network RADIUS SAS. Licenced under CC-by-NC 4.0.
// Development of this documentation was sponsored by Network RADIUS SAS.
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