SQL> create table test (col varchar2(20));

Table created.

SQL> insert all
  2    into test values ('no_space_in_here')
  3    into test values ('we have 3 spaces')
  4    into test values ('only one_space')
  5  select * from dual;

3 rows created.

SQL> select col,
  2    case
  3      when regexp_instr(col, '[url=/wiki/:blank:]:blank:[/url]') > 0 then
  4        'Yes, there is at least one space in here'
  5      else
  6        'No, no space in this string'
  7    end space_detector
  8  from test;

COL                  SPACE_DETECTOR
-------------------- ----------------------------------------
no_space_in_here     No, no space in this string
we have 3 spaces     Yes, there is at least one space in here
only one_space       Yes, there is at least one space in here

SQL>

when regexp_instr(col, '[url=/wiki/:blank:]:blank:[/url]') > 0 then