Be more precise in documentation of char set when escaping is needed

doc/lexer-rules.md

@@ -58,7 +58,7 @@ Match that character or sequence of characters. E.g., ’while’ or ’=’.</t
 
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 <td>[char set]</td><td>
-<p>Match one of the characters specified in the character set. Interpret <tt>x-y</tt> as the set of characters between range <tt>x</tt> and <tt>y</tt>, inclusively. The following escaped characters are interpreted as single special characters: <tt>\n</tt>, <tt>\r</tt>, <tt>\b</tt>, <tt>\t</tt>, <tt>\f</tt>, <tt>\uXXXX</tt>, and <tt>\u{XXXXXX}</tt>. To get <tt>]</tt>, <tt>\</tt>, or <tt>-</tt> you must escape them with <tt>\</tt>.</p>
+<p>Match one of the characters specified in the character set. Interpret <tt>x-y</tt> as the set of characters between range <tt>x</tt> and <tt>y</tt>, inclusively. The following escaped characters are interpreted as single special characters: <tt>\n</tt>, <tt>\r</tt>, <tt>\b</tt>, <tt>\t</tt>, <tt>\f</tt>, <tt>\uXXXX</tt>, and <tt>\u{XXXXXX}</tt>. To get <tt>]</tt> or <tt>\</tt> you must escape them with <tt>\</tt>. To get <tt>-</tt> you must escape it with <tt>\</tt> too, except for the case when <tt>-</tt> is the first or last character in the set.</p>
 
 <p>You can also include all characters matching Unicode properties (general category, boolean, or enumerated including scripts and blocks) with <tt>\p{PropertyName}</tt> or <tt>\p{EnumProperty=Value}</tt>. (You can invert the test with <tt>\P{PropertyName}</tt> or <tt>\P{EnumProperty=Value}</tt>).</p>
 
@@ -90,6 +90,8 @@ UNICODE_ID : [\p{Alpha}\p{General_Category=Other_Letter}] [\p{Alnum}\p{General_C
 EMOJI : [\u{1F4A9}\u{1F926}] ; // note Unicode code points > U+FFFF
 
 DASHBRACK : [\-\]]+ ; // match - or ] one or more times
+
+DASH : [---] ; // match a single -, i.e., "any character" between - and - (note first and last - not escaped)
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