SQL> ;
  1  with
  2   t(s) as (
  3      select 'wOrD1' from dual union all
  4      select 'wOrD1 WORD2' from dual union all
  5      select 'wOrD1 WORD2 WoRd3' from dual union all
  6      select 'wOrD1 WORD2 WoRd3 Word4 Word5' from dual union all
  7      select 'WoRd3' from dual union all
  8      select 'wOrDA wordB WoRd1 WordD wordE' from dual
  9   )
 10  ,words(n,word) as (
 11      select rownum,column_value from table(sys.odcivarchar2list('word1','word3'))
 12   )
 13  select
 14    t.s
 15    ,xt.str
 16  from t
 17      ,xmltable(
 18         'fn:replace(.,"(word1|word3)",">$1<","i")'
 19         passing t.s
 20         columns
 21           str varchar2(3000) path '.'
 22*     ) xt
SQL> /

S                                                  STR
-------------------------------------------------- --------------------------------------------------
wOrD1                                              >word1<
wOrD1 WORD2                                        >word1< WORD2
wOrD1 WORD2 WoRd3                                  >word1< WORD2 >wordord3<d1<
wOrD1 WORD2 WoRd3 Word4 Word5                      >word1< WORD2 >wordord3<d1< Word4 Word5
WoRd3                                              >word3<
wOrDA wordB WoRd1 WordD wordE                      wOrDA wordB >wordordord1< WordD wordE

SQL> ;
  1  with
  2   t(s) as (
  3      select 'wOrD1 WORD2 WoRd3 Word4 Word1' from dual
  4   )
  5  select
  6    t.s
  7    ,xt.str
  8  from t
  9      ,xmltable(
 10         'fn:replace(.,"(word)",">$1<","i")'
 11         passing t.s
 12         columns
 13           str varchar2(3000) path '.'
 14*     ) xt
SQL> /

S                                                  STR
-------------------------------------------------- ------------------------------------------------------------
wOrD1 WORD2 WoRd3 Word4 Word1                      >word<1 >word<d<2 >word<d<d<3 >word<d<d<d<4 >word<d<d<d<d<1

$ perl -e "print q{\lA\tA}.'===='.qq{\lB\tB\n}"
\lA\tA====b     B

Customary  Generic        Meaning        Interpolates
    ''       q{}          Literal             no
    ""      qq{}          Literal             yes
    ``      qx{}          Command             yes*
            qw{}         Word list            no
    //       m{}       Pattern match          yes*
            qr{}          Pattern             yes*
             s{}{}      Substitution          yes*
            tr{}{}    Transliteration         no (but see below)
             y{}{}    Transliteration         no (but see below)
    <<EOF                 here-doc            yes*

    * unless the delimiter is ''.

$ echo abcdef | perl -pe 's/(.)./\U\1/g'
ACE

$ echo abcdef | perl -pe '$r="\U\\1"; s/(.)./$r/g'
\1\1\1

$ echo abcdef | perl -pe '$r="\U\\1x"; s/(.)./\u$r/g'
\1X\1X\1X