Re: why matches($title,'.*?(\.|,)\s*$')) can perform s
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 '^'):
then I would expect perfomance to be very similar to:
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.
On 12/07/2011 17:26, Alex Muir wrote:
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