Fix parsing nul bytes in regex

Fixes #4460

src/regex_impl.cc

@@ -340,7 +340,7 @@ private:
             case '|': case ')':
                 return {};
             default:
-                if (contains("^$.*+?[]{}", cp) or (cp >= 0xF0000 and cp <= 0xFFFFF))
+                if (contains(StringView{"^$.*+?[]{}"}, cp) or (cp >= 0xF0000 and cp <= 0xFFFFF))
                     parse_error(format("unexpected '{}'", cp));
                 ++m_pos;
                 return add_node(ParsedRegex::Literal, cp);

test/regression/4460-nul-byte-in-regex/cmd

@@ -0,0 +1 @@
+*%s<ret>d

test/regression/4460-nul-byte-in-regex/out

@@ -0,0 +1 @@
+