Oracle 11g Release 2

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create table myclobs ( col clob ) ;
insert into myclobs ( col )
values ( '<b>Hello world</b><script>window.alert(''Hello world'')</script>' );
1 rows affected

select * from myclobs ;


COL


<b>Hello world</b><script>window.alert('Hello world')</script>


begin
  insert into myclobs ( col )
    values ( '<b>Hello again!</b><script>window.alert(''Hello again!'')</script>');
  insert into myclobs ( col )
    values ( '<b>Yet another hello!</b><script>window.alert(''Yet another hello'')</script>');
end ;
/
1 rows affected

-- insert a bigger CLOB
begin
  insert into myclobs ( col )
  values ( '<b>Hello world</b><script>window.alert(''Hello world'')</script><b>Hello again!</b><script>window.alert(''Hello again!'')</script><b>Yet another hello!</b><script>window.alert(''Yet another hello'')</script>' )
  ;
end ;
/
1 rows affected

select * from myclobs ;


COL


<b>Hello world</b><script>window.alert('Hello world')</script>

<b>Hello again!</b><script>window.alert('Hello again!')</script>

<b>Yet another hello!</b><script>window.alert('Yet another hello')</script>

<b>Hello world</b><script>window.alert('Hello world')</script><b>Hello again!</b><script>window.alert('Hello again!')</script><b>Yet another hello!</b><script>window.alert('Yet another hello')</script>


select
  replace( 
    replace( col, '</script>','&lt;/script&gt;' )
  , '<script>','&lt;script&gt;' 
  ) as scripttagsreplaced
from myclobs ;


SCRIPTTAGSREPLACED


<b>Hello world</b>&lt;script&gt;window.alert('Hello world')&lt;/script&gt;

<b>Hello again!</b>&lt;script&gt;window.alert('Hello again!')&lt;/script&gt;

<b>Yet another hello!</b>&lt;script&gt;window.alert('Yet another hello')&lt;/script&gt;

<b>Hello world</b>&lt;script&gt;window.alert('Hello world')&lt;/script&gt;<b>Hello again!</b>&lt;script&gt;window.alert('Hello again!')&lt;/script&gt;<b>Yet another hello!</b>&lt;script&gt;window.alert('Yet another hello')&lt;/script&gt;


create or replace function rst( theclob clob )
return clob
is
begin  
  return  replace( 
    replace( theclob, '</script>','&lt;/script&gt;' ) 
  , '<script>','&lt;script&gt;' 
  ) ;
end ;
/
select rst( col ) from myclobs ;


RST(COL)


<b>Hello world</b>&lt;script&gt;window.alert('Hello world')&lt;/script&gt;

<b>Hello again!</b>&lt;script&gt;window.alert('Hello again!')&lt;/script&gt;

<b>Yet another hello!</b>&lt;script&gt;window.alert('Yet another hello')&lt;/script&gt;

<b>Hello world</b>&lt;script&gt;window.alert('Hello world')&lt;/script&gt;<b>Hello again!</b>&lt;script&gt;window.alert('Hello again!')&lt;/script&gt;<b>Yet another hello!</b>&lt;script&gt;window.alert('Yet another hello')&lt;/script&gt;