update README for variable substitution feature

Author: zazedd

README.md

@@ -26,12 +26,23 @@ function%pcre
 into suitable invocations of the [Re library][re], and similar for
 `match%pcre`.  The patterns are plain strings of the form accepted by
 `Re_pcre`, with the following additions:
-
+    
   - `(?<var>...)` defines a group and binds whatever it matches as `var`.
     The type of `var` will be `string` if the match is guaranteed given that
     the whole pattern matches, and `string option` if the variable is bound
     to or nested below an optionally matched group.
 
+  - `(N?<var>)` gets substituted by the value of the `%pcre` extended string variable named `var`,
+    and binds whatever it matches as `var`.
+    The type of `var` will be the same as `(?<var>...)`.
+
+  - `(N?<var as name>)` gets substituted by the value of the `%pcre` extended string variable named `var`,
+    and binds whatever it matches as `name`.
+    The type of `name` will be the same as `(?<var>...)`.
+
+  - `(U?<var>)` gets substituted by the value of the globally defined string variable named `var`,
+    and does not bind its match to any name.
+
   - `?<var>` at the start of a pattern binds group 0 as `var : string`.
     This may not be the full string if the pattern is unanchored.