Oracle 11g Release 2

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DECLARE
  PROCEDURE testClob(
    p_position NUMBER,
    p_value    VARCHAR2
  )
  IS
    p_clob   CLOB         := '<root><a>';
    p_middle VARCHAR2(7)  := '</a><b>';
    p_close  VARCHAR2(11) := '</b></root>';
    p_deduction NUMBER    := LENGTH( p_clob ) + LENGTH( p_middle );
    p_a     CLOB;
    p_b     VARCHAR2(10);
  BEGIN
    FOR i IN 1 .. FLOOR( ( p_position - p_deduction) / 4000 ) LOOP
      p_clob := p_clob || RPAD( '_', 4000, '_' );
    END LOOP;
    p_clob := p_clob || RPAD( '_', MOD( p_position - p_deduction, 4000 ), '_' );
    p_clob := p_clob || p_middle;
    DBMS_OUTPUT.PUT( LENGTH( p_clob ) || ' characters then "' || p_value || '" gives ' );
    p_clob := p_clob || p_value;
    p_clob := p_clob || p_close;

SELECT a, b
    INTO   p_a, p_b
    FROM   XMLTABLE(
             '/root'
             PASSING XMLTYPE( p_clob )
             COLUMNS
               a CLOB         PATH 'a',
               b VARCHAR2(10) PATH 'b'
           );
  
    DBMS_OUTPUT.PUT( '"'||p_b||'" (XMLTABLE)' );
    
    SELECT EXTRACTVALUE( XMLTYPE( p_clob ), '/root/b' )
    INTO   p_b
    FROM   DUAL;

DBMS_OUTPUT.PUT( ' or "'||p_b||'" (EXTRACTVALUE)' );

SELECT XMLQUERY(
             '/root/b/text()'
             PASSING XMLTYPE( p_clob )
             RETURNING CONTENT
           ).getClobVal()
    INTO   p_b
    FROM   DUAL;

DBMS_OUTPUT.PUT_LINE( ' or "'||p_b||'" (XMLQUERY)' );
  END;
BEGIN
  testClob( 39998, '' );
  testClob( 39998, ' ' );
  testClob( 39998, '  ' );
  testClob( 39998, 'ABC' );
  testClob( 39998, 'A' );
END;
/

1 rows affected

dbms_output:
39998 characters then "" gives "" (XMLTABLE) or "" (EXTRACTVALUE) or "" (XMLQUERY)
39998 characters then " " gives "" (XMLTABLE) or "" (EXTRACTVALUE) or "" (XMLQUERY)
39998 characters then "  " gives "  " (XMLTABLE) or "  " (EXTRACTVALUE) or "  " (XMLQUERY)
39998 characters then "ABC" gives "ABC" (XMLTABLE) or "ABC" (EXTRACTVALUE) or "ABC" (XMLQUERY)
39998 characters then "A" gives "A" (XMLTABLE) or "A" (EXTRACTVALUE) or "A" (XMLQUERY)