fix ^* at the start of a complete BRE

This is a workaround to treat * as literal * at the start of a BRE.

Ideally ^ would be treated as an anchor at the start of any BRE
subexpression and similarly $ would be an anchor at the end of any
subexpression.  This is not required by the standard and hard to do
with the current code, but it's the existing practice.  If it is
changed, * should be treated as literal after such anchor as well.

src/regex/regcomp.c

@@ -994,6 +994,10 @@ static reg_errcode_t tre_parse(tre_parse_ctx_t *ctx)
 			if (*s=='\\')
 				s++;
 
+			/* handle ^* at the start of a complete BRE. */
+			if (!ere && s==ctx->re+1 && s[-1]=='^')
+				break;
+
 			/* extension: multiple consecutive *+?{,} is unspecified,
 			   but (a+)+ has to be supported so accepting a++ makes
 			   sense, note however that the RE_DUP_MAX limit can be