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+--- Thread: Basic Negate Question (/showthread.php?tid=1677)
Basic Negate Question - qz33 - Oct. 12, 2010 12:43 PM
I am trying to understand the negate function in the text matching language. I have never tried to write a filter using it before other than a single character negation.
Why does
*(^arstechnica)*
match when given http://www.arstechnica.com
Below is the only reference from the Proxomitron help files that directly relates too a whole word negation :
"For example, "(^foo|bar)" would match anything that's not "foo" or "bar". Note that a negated expression consumes no characters - it just test them. I think Perl calls this a "negative forward assertion"?"
Does anyone have enlightenment?
I am trying to blank out any urls that are NOT something.arstechnica.something.
Even if there is a better way to do this I need to understand why the above match expression fails and provides a match when I think it should not.
RE: Basic Negate Question - JJoe - Oct. 12, 2010 03:15 PM
(Oct. 12, 2010 12:43 PM)qz33 Wrote: Why does
*(^arstechnica)*
match when given http://www.arstechnica.com
The leading wildcard can match nothing.
(^arstechnica)* matches
HTH
RE: Basic Negate Question - whenever - Oct. 12, 2010 03:31 PM
(^foo) matches a position, where it is not followed by "foo".
Try below filter, which puts the matched result of the first * into \1, (^arstechnica) into \2 and the last * into \3.
Code:
[Patterns]
Name = "Test filter"
Active = FALSE
Limit = 256
Match = "\1((^arstechnica))\2\3"
Replace = "\\1 is: \1\r\n"
"\\2 is: \2\r\n"
"\\3 is: \3\r\n"When given http://www.arstechnica.com, it outputs:
Quote:\1 is:
\2 is:
\3 is: http://www.arstechnica.com
The first * started from matching zero character, that is, a position before the first h.
At that position, it is not followed by "arstechnica", so (^arstechnica) matched
The last * matched all left characters
...
So the whole expression matched.
HTH.
RE: Basic Negate Question - qz33 - Oct. 12, 2010 06:17 PM
Great so far! Thanks.
So I tried this
(src|href)="http://*.(^arstechnica).*
against href="http://static.arstechnica.com/apple/ (yes I know it is missing a closing quotation)
and it did not match which is what I wanted and expected.
but when I put the " . " in the negated sub-expression like this
(src|href)="http://*.(^arstechnica.)*
it matches against href="http://static.arstechnica.com/apple/
which again I think it should not but I do not understand why as it seems that it would match up to the FIRST " . " then everything after IS "arstechnica.com/apple/" which should not match the because of the (^arstechnica.) part.
So can someone help with the why here
I guess I really should be using (src|href)"http://\0.(^arstechnica.)\1\2
RE: Basic Negate Question - JJoe - Oct. 13, 2010 12:30 AM
If I understand correctly,
Code:
(src|href)="http://*.(^arstechnica).*(^arstechnica) doesn't actually 'consume' anything. It just checks to see if arstechnica is not there.
So, the part of the expression that 'consumes' is (src|href)="http://*..* and it doesn't match much.
Code:
(src|href)="http://*.(^arstechnica.)*
against
href="http://static.arstechnica.com/apple/The (src|href)="http://*.(^arstechnica.) part does match href="http://static.arstechnica. as arstechnica. is not followed by arstechnica.
*.(^arstechnica.) didn't stop looking for a match after static. was found.
Code:
http://\0.(^arstechnica.)\1\2This will fail when every period is followed by "arstechnica.". Probably not going to happen.
\1 will always be empty.
Why do these match http://static.rstechnica.com/apple/ but not http://static.arstechnica.com/apple/ or do they?
http://(^*.arstechnica)*
http://([^.]+.(^arstechnica))+{1,*}*
http://((*.)+{1}(^arstechnica))+{1,*}*
HTH
RE: Basic Negate Question - qz33 - Oct. 13, 2010 02:35 AM
Ok it is slowly sinking in now. I believe I am not being strict enough with the meaning of the operator symbols.
Man is this just a retard question for you guys? I really do appreciate it.