regex clarifications

Consistently "quote" examples.
Split complemented simple and range class examples.
Improve "(" .. ")" grouping example.

Author: lukem

jsonschema-core.md

@@ -400,14 +400,15 @@ tokens:
 
 - individual Unicode characters, as defined by the [JSON
   specification](#rfc8259);
-- simple character classes ([abc]), range character classes ([a-z]);
-- complemented character classes ([^abc], [^a-z]);
+- simple character classes ("[abc]"), range character classes ("[a-z]");
+- complemented simple character classes ("[^abc]"),
+  complemented range character classes ("[^a-z]");
 - simple quantifiers: "+" (one or more), "*" (zero or more), "?" (zero or one),
   and their lazy versions ("+?", "*?", "??");
 - range quantifiers: "{x}" (exactly x occurrences), "{x,y}" (at least x, at most
   y, occurrences), {x,} (x occurrences or more), and their lazy versions;
 - the beginning-of-input ("^") and end-of-input ("$") anchors;
-- simple grouping ("(...)") and alternation ("|").
+- simple grouping ("(" and ")") and alternation ("|").
 
 Finally, implementations MUST NOT take regular expressions to be anchored,
 neither at the beginning nor at the end. This means, for instance, the pattern