Oracle 18c

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select 
  'to_char( 1,''9990'' )'       function_
, dump( to_char( 1,'9990' ) )   dump_
, to_char( 1, '9990' )          output_
from dual union all
select 'to_char( 1,''9999'' )', dump( to_char( 1, '9999' ) )
, to_char( 1, '9999' )
from dual union all 
select 'to_char( 1 )', dump( to_char( 1 ) )
, to_char( 1 )
from dual union all 
select 'to_char( 1,''fm9990'' )', dump( to_char( 1,'fm9990' ) )
, to_char( 1, '9999' )
from dual union all 
select 'to_char( 1,''fm9999'' )', dump( to_char( 1,'fm9999' ) )
, to_char( 1, '9999' )
from dual union all 
select 'to_char( 0,''99.90'' )', dump( to_char( 0,'99.90' ) )
, to_char( 0, '99.90' )  
from dual union all
select 'to_char( 0,''99.99'' )', dump( to_char( 0,'99.99' ) )
, to_char( 0, '99.99' )  
from dual union all
select 'to_char( 0,''90.99'' )', dump( to_char( 0,'90.99' ) )
, to_char( 0, '90.99' )  
from dual union all
select 'to_char( 0,''fm99.90'' )', dump( to_char( 0,'fm99.90' ) )
, to_char( 0, 'fm99.90' )  
from dual union all
select 'to_char( 0,''fm99.99'' )', dump( to_char( 0,'fm99.99' ) )
, to_char( 0, 'fm99.99' )  
from dual union all
select 'lpad( ''1'', 5, '' '' )', null, lpad( '1', 5, ' ' ) from dual union all
select 'lpad( ''12345'', 6, ''_'' )', null, lpad( '12345', 6, '_' ) from dual union all
select 'rpad( ''12345'', 6, ''_'' )', null, rpad( '12345', 6, '_' ) from dual
;

FUNCTION_	DUMP_	OUTPUT_
to_char( 1,'9990' )	Typ=1 Len=5: 32,32,32,32,49	1
to_char( 1,'9999' )	Typ=1 Len=5: 32,32,32,32,49	1
to_char( 1 )	Typ=1 Len=1: 49	1
to_char( 1,'fm9990' )	Typ=1 Len=1: 49	1
to_char( 1,'fm9999' )	Typ=1 Len=1: 49	1
to_char( 0,'99.90' )	Typ=1 Len=6: 32,32,32,46,48,48	.00
to_char( 0,'99.99' )	Typ=1 Len=6: 32,32,32,46,48,48	.00
to_char( 0,'90.99' )	Typ=1 Len=6: 32,32,48,46,48,48	0.00
to_char( 0,'fm99.90' )	Typ=1 Len=3: 46,48,48	.00
to_char( 0,'fm99.99' )	Typ=1 Len=2: 48,46	0.
lpad( '1', 5, ' ' )	null	1
lpad( '12345', 6, '_' )	null	_12345
rpad( '12345', 6, '_' )	null	12345_