(ebnf) add backtick (grave symbol) as string character (#2290)

This adds backticks as an allowed character for terminal strings.

This has precedent in some EBNF-described specifications such as the Go
specification.

CHANGES.md

@@ -7,6 +7,7 @@ New styles:
   none.
 
 Improvements:
+- enh(ebnf) add backticks as additional string variant (#2290) [Chris Marchesi][]
 - fix(themes): fix inconsistencies between some themes padding/spacing (#2300) [Josh Goebel][]
 - chore(javascript): add esm related extensions to aliases (#2298) [Rongjian Zhang][]
 - fix(kotlin): fix termination of """ string literals (#2295) [Josh Goebel][]

src/languages/ebnf.js

@@ -22,8 +22,15 @@ function(hljs) {
         contains: [
             commentMode,
             specialSequenceMode,
-            // terminals
-            hljs.APOS_STRING_MODE, hljs.QUOTE_STRING_MODE
+            {
+              // terminals
+              className: 'string',
+              variants: [
+                hljs.APOS_STRING_MODE,
+                hljs.QUOTE_STRING_MODE,
+                {begin: '`', end: '`'},
+              ]
+            },
         ]
     };
 

test/markup/ebnf/quote-symbols.expect.txt

@@ -0,0 +1,8 @@
+<span class="hljs-attribute">first_quote_symbol</span>  = <span class="hljs-string">"'"</span> .
+<span class="hljs-attribute">second_quote_symbol</span> = <span class="hljs-string">'"'</span> .
+
+<span class="hljs-comment">(* escaped_quote_symbol tests backticks, which does not interfere
+ * with backslashes. It has precedent in some language
+ * specifications.
+ *)</span>
+<span class="hljs-attribute">escaped_quote_symbol</span> = <span class="hljs-string">`\`</span> ( first_quote_symbol | second_quote_symbol ) .

test/markup/ebnf/quote-symbols.txt

@@ -0,0 +1,8 @@
+first_quote_symbol  = "'" .
+second_quote_symbol = '"' .
+
+(* escaped_quote_symbol tests backticks, which does not interfere
+ * with backslashes. It has precedent in some language
+ * specifications.
+ *)
+escaped_quote_symbol = `\` ( first_quote_symbol | second_quote_symbol ) .