regexp in SQL

SELECT REGEXP_SUBSTR('Go to http://www.oracle.com/products and click on database',
'http://([[:alnum:]]+\.?){3,4}/?') RESULT
FROM dual;
Can some body explain what this is doing?
output : http://www.oracle.com/


What exactly +, \. , \n  do .
Thanks
avi_nyAsked:
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paquicubaConnect With a Mentor Commented:
>>> What exactly +, \. , \n  do .

+   Matches one or more occurrences of the preceding subexpression

\.  Escape Character:  Treat the subsequent metacharacter in the expression as a literal.
                                 Since "." ( dot ) has a special meaning within the context of a regular expression, you
                                 must use "\" (Escape Character) to treat "." (dot) as a literal.

\n  Backreference:  Matches the nth preceding subexpression, where n is an integer from 1 to 9.
 
 
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actonwangCommented:
>>http://([[:alnum:]]+\.?){3,4}/?

     it is a reg expression.

     it matches.
     http://  then 3 or 4 set of   alphanumeric charachters ending with . (\. because . is a special character) plus a '/' (optional because of ?).
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