CLDR-16249 Describe EBNF syntax more clearly (#3322)

docs/ldml/tr35.md

@@ -246,7 +246,13 @@ External specifications may also reference particular components of Unicode loca
 
 > _Field X can contain any Unicode region subtag values as given in Unicode Technical Standard #35: Unicode Locale Data Markup Language (LDML), excluding grouping codes._
 
+### EBNF
+The BNF syntax used in LDML is a variant of the Extended Backus-Naur Form (EBNF) notation used in [W3C XML Notation](https://www.w3.org/TR/REC-xml/#sec-notation). The main differences are:
 
+1. Bounded repetition following Perl regex syntax is allowed, such as alphanum{3,8}
+2. Constraints (well-formedness or validity) use separate notes
+
+In the text, this is sometimes referred to as "EBNF (Perl-based)".
 
 ## <a name="Locale" href="#Locale">What is a Locale?</a>