diff --git a/spec/message.abnf b/spec/message.abnf
index 30c3415584..278402d1bc 100644
--- a/spec/message.abnf
+++ b/spec/message.abnf
@@ -36,11 +36,12 @@ quoted-char = %x0-5B         ; omit \
             / %x7D-D7FF      ; omit surrogates
             / %xE000-10FFFF
 
-; based on https://www.w3.org/TR/xml/#NT-Nmtoken,
-; but cannot start with U+002D HYPHEN-MINUS or U+003A COLON ":"
+; literals that do not require quoting
+; containing no whitespace and with a restricted range of characters
 unquoted = unquoted-start *name-char
 unquoted-start = name-start / DIGIT / "."
                / %xB7 / %x300-36F / %x203F-2040
+               / "-" ( DIGIT / "." )
 
 
 ; reserve sigils for private-use by implementations
diff --git a/spec/syntax.md b/spec/syntax.md
index 099d5a4696..5492bd5821 100644
--- a/spec/syntax.md
+++ b/spec/syntax.md
@@ -605,9 +605,10 @@ Implementations MUST NOT distinguish between _quoted_ and _unquoted_ literals
 that have the same sequence of code points.
 
 _Unquoted_ literals have a much more restricted range that
-is intentionally close to the XML's [Nmtoken](https://www.w3.org/TR/xml/#NT-Nmtoken),
-with the restriction that it MUST NOT start with `-` or `:`,
-as those would conflict with _function_ start characters.
+is intentionally close to XML's [Nmtoken](https://www.w3.org/TR/xml/#NT-Nmtoken).
+To make _unquoted_ literals distinct from _function_ names,
+a literal MUST be quoted if it begins with a `:`
+or if it begins with a `-` that is **not** followed by a `.` or a digit.
 
 ```abnf
 literal = quoted / unquoted
@@ -621,6 +622,7 @@ quoted-char    = %x0-5B         ; omit \
 unquoted       = unquoted-start *name-char
 unquoted-start = name-start / DIGIT / "."
                / %xB7 / %x300-36F / %x203F-2040
+               / "-" ( DIGIT / "." )
 ```
 
 ### Names