Oracle 11g Release 2

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CREATE TABLE tab( col CLOB )
INSERT INTO tab
VALUES('[VARIABLE_NAME_ONE] variable_one = 1 + variable_two;
[VARIABLE_NAME_TWO] variable_two = 2 + variable_three;
[VARIABLE_NAME_ONE] variable_one = variable_four - 4;')
1 rows affected

UPDATE tab 
   SET col = (
              WITH t AS
              (
                 SELECT REPLACE(REGEXP_SUBSTR(col,'[^;]+',1,level),']','_'||level||']') 
                     AS col, level AS lvl
                  FROM TAB t
                CONNECT BY level <= REGEXP_COUNT(col,';')
              )
              SELECT LISTAGG(col,';') WITHIN GROUP (ORDER BY lvl)||';'
                FROM t)
1 rows affected

SELECT * FROM tab


COL


[VARIABLE_NAME_ONE_1] variable_one = 1 + variable_two;
[VARIABLE_NAME_TWO_2] variable_two = 2 + variable_three;
[VARIABLE_NAME_ONE_3] variable_one = variable_four - 4;