SQL> create table tbl(a varchar2(10),b varchar2(10))
  2  /

Table created.

SQL> create or replace
  2    trigger tbl_biur
  3      before insert
  4      or update
  5      on tbl
  6      for each row
  7      begin
  8          IF :new.a='AAA' AND :new.b NOT LIKE '/%/'
  9            THEN
 10              RAISE_APPLICATION_ERROR(-20002,'Must begin and end with /');
 11          END IF;
 12  end;
 13  /

Trigger created.

SQL> insert
  2    into tbl
  3    values(
  4           'AAA',
  5           '/BBB/'
  6          )
  7  /

1 row created.

SQL> insert
  2    into tbl
  3    values(
  4           'AAA',
  5           'CCC'
  6          )
  7  /
  into tbl
       *
ERROR at line 2:
ORA-20002: Must begin and end with /
ORA-06512: at "SCOTT.TBL_BIUR", line 4
ORA-04088: error during execution of trigger 'SCOTT.TBL_BIUR'


SQL> update tbl
  2     set b = 'CCC'
  3  /
update tbl
       *
ERROR at line 1:
ORA-20002: Must begin and end with /
ORA-06512: at "SCOTT.TBL_BIUR", line 4
ORA-04088: error during execution of trigger 'SCOTT.TBL_BIUR'


SQL>