Description:
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I was testing some input being passed via a parameter to my script.  I was getting an error indicating that the value passed was invalid, yet it didn't contain any of the strings I was searching for.  
I went through the process of shortening the string and when I got below 8192 chars in length it worked, over 8192 chars in length it would fail.

I also tried this on PHP v7.1.32 and the exact same code on my Mac OSX which worked correctly.

Test script:
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filter_var(trim($_REQUEST['parms']), FILTER_VALIDATE_REGEXP, array("options" => array("regexp" => "/^((?!union select|union all|\'A=|select char).)*$/im")));

Expected result:
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A) filter_var should handle strings longer than 8192 chars

OR

B) Add documentation to filter_var manual entry indicating maximum length of string.