Oracle 18c

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create table test (xmldata) as
select xmltype('<workbook>
 <worksheet sheetName="asd-kasd" sheetId="1"/>
</workbook>') 
from dual;
1 rows affected

select * from test;


XMLDATA


<workbook>
  <worksheet sheetName="asd-kasd" sheetId="1"/>
</workbook>



update test
set XMLDATA=
           xmlquery(
             'copy $d := .
             modify (
                for $i in $d/workbook/worksheet/@sheetName
                return replace value of node $i with concat("1.\w:",replace(string($i/../@sheetName),"\w",""),"2. \W:",replace($i/../@sheetName,"\W",""))
            )
              return $d'
            passing test.XMLDATA
            returning content
         )
ORA-19112: error raised during evaluation: 
XVM-01126: [FORX0003] Regular expression matches zero-length string

select * from test;


XMLDATA


<workbook>
  <worksheet sheetName="asd-kasd" sheetId="1"/>
</workbook>