psql treat backslash which is not the first character in the input line as meta-command

Hi,all

psql treat backslash which is not the first character in the input line as meta-command,and failed to execute the next query.
It seems to be a bug.

Sample1:
postgres=# select \1 set c2=c1%3 ;
Invalid command \1. Try \? for help.
postgres-# select 1;
ERROR: syntax error at or near "select"
LINE 2: select 1;
^

Sample2:
postgres=# a \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-----------+--------+-----------+------------+------------+-------------------
postgres | chenhj | SQL_ASCII | C | C |
template0 | chenhj | SQL_ASCII | C | C | =c/chenhj +
| | | | | chenhj=CTc/chenhj
template1 | chenhj | SQL_ASCII | C | C | =c/chenhj +
| | | | | chenhj=CTc/chenhj
(6 rows)

postgres-# select 1;
ERROR: syntax error at or near "a"
LINE 1: a
^

Regards,
Chen Huajun