sql > select * from v$version where rownum=1;

BANNER
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Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

sql > create function
  2    eval_simple_arithmetic_expr(vexpr in varchar2)
  3    return number
  4  is
  5  evaluated number;
  6  begin
  7  select
  8    xmlquery(
  9    replace( vexpr, '/', ' div ')
 10       returning content
 11    ).getNumberVal()
 12  into evaluated
 13  from
 14  dual;
 15  return evaluated;
 16  end eval_simple_arithmetic_expr;
 17  /

Function created.

sql > select eval_simple_arithmetic_expr('5/100*300'), 5/100*300 from dual;

EVAL_SIMPLE_ARITHMETIC_EXPR('5/100*300')  5/100*300
---------------------------------------- ----------
                                      15         15

sql > select eval_simple_arithmetic_expr('1*2*3+4+5*6+7'), 1*2*3+4+5*6+7 from dual;

EVAL_SIMPLE_ARITHMETIC_EXPR('1*2*3+4+5*6+7') 1*2*3+4+5*6+7
-------------------------------------------- -------------
                                          47            47

sql > select eval_simple_arithmetic_expr('193 mod 47'), mod(193, 47) from dual;

EVAL_SIMPLE_ARITHMETIC_EXPR('193MOD47') MOD(193,47)
--------------------------------------- -----------
                                      5           5