--Query 1

SELECT CASE WHEN 
REGEXP_LIKE( '5 SEC IN 20.0 MIN', '^[0-9]*[0-9] SEC IN [0-9]*[0-9]\.[0-9]*[0-9] MIN$') THEN 'TRUE' 
ELSE 'NULL'
END
FROM dual;

--Query 2

SELECT CASE WHEN 
REGEXP_LIKE( '0.55 SEC IN 200.0 MIN', '^[0-9]*[0-9]|\.[0-9]*[0-9] SEC IN [0-9]*[0-9]\.[0-9]*[0-9] MIN$') THEN 'TRUE' 
ELSE 'NULL'
END
FROM dual;

with tst as (select '5 SEC IN 20.0 MIN' str from dual
             union all
             select '0.55 SEC IN 200.0 MIN' from dual
             union all
             select '0.5 200 NO-SEC IN 20.0 MIN' from dual)
select str,
       case
          when regexp_like(str, '^\d+\.?\d* SEC IN \d+\.?\d* MIN$') then
             'true'
       else
          'false'
       end   
from tst

^ - start of pattern

\d - number of any length

+  - plus additional pattern

\.?\d - now includes decimals

* - The zero or more operator '*', matches zero or more occurrences of the preceding character or subexpression