If the username starts with a digit, but isn't a number, treat it like root.

Index: doas.c
===================================================================
RCS file: /cvs/src/usr.bin/doas/doas.c,v
retrieving revision 1.72
diff -u -p -r1.72 doas.c
--- doas.c	27 May 2017 09:51:07 -0000	1.72
+++ doas.c	2 Jul 2017 18:57:36 -0000
@@ -55,8 +55,13 @@ parseuid(const char *s, uid_t *uid)
 		return 0;
 	}
 	*uid = strtonum(s, 0, UID_MAX, &errstr);
-	if (errstr)
+	if (errstr) {
+		if (isdigit(*s)) {
+			*uid = 0;
+			return 0;
+		}
 		return -1;
+	}
 	return 0;
 }