Re: [xsl] why matches($title,'.*?(\.|,)\s*$')) can perform so much worse than matches($title,'(\.|,)\s*$'))
2011-07-13 07:36:49
The way that fn:matches usually works is that it attempts to apply the
regular expression starting from each character in the string.
So, looking at the regular expression ".*?Expr" where Expr is another
regular expression:
.*? is a reluctant match and so we try to match the shortest expression
possible first.
Starting from the first character:
Try match the empty string for .*? and then try to mach Expr starting
from the first character.
Try to match a single character for .*? and then try to match Expr
starting from the second character.
Try to match two characters for .*? and then try to match Expr
starting from the third character.
...
Thus in trying to find a match for ".*?Expr" starting at a particular
character we try to match Expr starting at every other caharcter in the
string, until a match is found.
fn:matches attempts to find a match for the regular expression starting
from each character in the string. If a match isnt found starting at
the first character then it will try again starting at the second and so
on. So if there are no matches then Expr is tested n^2 times where n is
the number of characters in the string, wheras matches applied to Expr
would just test it n times.
Now, if your query was (note the '^'):
matches($title,'^.*?(\.|,)\s*$')
then I would expect perfomance to be very similar to:
matches($title,'(\.|,)\s*$')
It would be perfectly valid (and sensible) for a query processor to
realise that the two expressions you gave were equivalent and so not
perform n^2 tests, but I am unaware of a processor that makes these
kinds of optimizations to regular expressions.
Oliver Hallam
XQSharp
On 12/07/2011 17:26, Alex Muir wrote:
Hi,
I'm wondering why matches($title,'.*?(\.|,)\s*$')) can perform so much
worse than matches($title,'(\.|,)\s*$'))
I found at least in one file out of thousands that I process the first
one can take a good 30 minutes to complete and the second is quick.
No doubt the .*? causes some problem but what exactly is the problem
that is causes?
Regards
--
Alex Muir
Instructor | Program Organizer - University Technology Student Work
Experience Building
University of the Gambia
http://sites.utg.edu.gm/alex/
Low budget software development benefiting development in the Gambia,
West Africa
Experience of a lifetime, come to Gambia and Join UTSWEB -
http://sites.utg.edu.gm/utsweb/
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