Some markup extensions are built in to XAML; that is, they are a part of the native XAML vocabulary. These are not specific to the WPF implementation of XAML; they are features of XAML as a language and are implemented in the System.Xaml assembly.
The built-in XAML markup extensions are typically prefixed with x: and enclosed within braces, like other markup extensions. However, not all language features that start with x: are markup extensions. For instance, the x:Class name that you use with an element is an attribute, not a markup extension. Similarly, x:Key and x:Name are attributes.
There are no built-in markup extensions for string manipulation in XAML.
Some of the built-in XAML markup extensions are described ...