diff --git a/src/oas.md b/src/oas.md
index 4106fbc320..2ffed2bc76 100644
--- a/src/oas.md
+++ b/src/oas.md
@@ -51,7 +51,7 @@ path-template                  = "/" *( path-segment "/" ) [ path-segment ]
 path-segment                   = 1*( path-literal / template-expression )
 path-literal                   = 1*pchar
 template-expression            = "{" template-expression-param-name "}"
-template-expression-param-name = 1*( %x00-79 / %x7C / %x7E-10FFFF ) ; every UTF8 character except { and }
+template-expression-param-name = 1*( %x00-7A / %x7C / %x7E-10FFFF ) ; every UTF8 character except { and }
 
 pchar               = unreserved / pct-encoded / sub-delims / ":" / "@"
 unreserved          = ALPHA / DIGIT / "-" / "." / "_" / "~"
@@ -575,6 +575,31 @@ servers:
 
 An object representing a Server Variable for server URL template substitution.
 
+The server URL templating is defined by the following [ABNF](https://tools.ietf.org/html/rfc5234) syntax.
+
+```abnf
+server-url-template  = 1*( literals / server-variable )
+server-variable      = "{" server-variable-name "}"
+server-variable-name = 1*( %x00-7A / %x7C / %x7E-10FFFF ) ; every UTF8 character except { and }
+
+literals       = 1*( %x21 / %x23-24 / %x26-3B / %x3D / %x3F-5B
+               / %x5D / %x5F / %x61-7A / %x7E / ucschar / iprivate
+               / pct-encoded)
+                    ; any Unicode character except: CTL, SP,
+                    ;  DQUOTE, "%" (aside from pct-encoded),
+                    ;  "<", ">", "\", "^", "`", "{", "|", "}"
+pct-encoded    =  "%" HEXDIG HEXDIG
+ucschar        =  %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
+               /  %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
+               /  %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
+               /  %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
+               /  %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
+               /  %xD0000-DFFFD / %xE1000-EFFFD
+iprivate       =  %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
+```
+
+Here, `literals`, `pct-encoded`, `ucschar` and `iprivate` definitions are taken from [RFC 6570](https://www.rfc-editor.org/rfc/rfc6570), incorporating the corrections specified in [Errata 6937](https://www.rfc-editor.org/errata/eid6937) for `literals`.
+
 ##### Fixed Fields
 
 | Field Name | Type | Description |