@RickR2H
Just my opinion, but reading over your PR I don't think your addressing the issue correctly. You have attempted to deal with the array to string conversion in one CASE; but reading the "clean" method docblock, all of the $type cases are affected any time the $source variable is an array (as "intended" by the docblock var definition for the $source)

Since $source is intended to be a string or an array of strings, any fix should address all $type cases rather than just the singular case in your PR. I'm not sure what the best why to deal with that is, but it looks like maybe creating a new variable that is only the first string of the $source variable to run through the $type switch case.

Maybe something like this pseudo-code before the switch case

// Are we dealing with an array or a string for the $source?
if (is_array($source))
{
// Get the first string value of the $source array and assign it to a new (string) variable for `preg_match()`
}
else
{
// The new variable = $source when it's just a string
}

Then change all the references in the cases that need a string value for preg_match() to use the new variable rather than $source. The exception is to not change the default case since it correctly deals with $source as is.

Another way to address this would be to consider changing preg_match() for preg_grep() since preg_grep() accepts an input array. Or to specify that we only want the first value from $source when at each preg_match() case. but as I said, I'm not sure what the best why to deal with this issue is.

PS I think this might be a framework issue rather than a CMS issue. I would suggest a PR in the framework for the input filter clean function https://github.com/joomla-framework/filter
Then the change can be back ported from the framework to the CMS

It looks like there is a fair amount of work needed on the clean function.

I gave it a shot, turns out to be a bit more complicated. You can check out my attempt https://github.com/photodude/filter/blob/patch-2/src/InputFilter.php

With the additional checks and function calls, I ended up with a single data scenario that fails. (an array with arrays for data)
https://travis-ci.org/joomla-framework/filter/jobs/70484294

Check com_tags, the error is most likely there and not in the filter class.

The thing is that com_tags is very strange in that the id parameter can either contain an integer (what we are used to), an array of integer (probably your case with multiple tags) or even a coma-separated list of ids.
I'm betting the issue is somewhere there

@Bakual I believe this is an issue in framework filter clean() method. I have fixed my PR that should address this issue.

joomla-framework/filter#13

@RickR2H could you test my PR in the Framework Filter library to see if this resolves the warnings you were seeing?

joomla-framework/filter#13

I have tested the steps listed in this issue with Joomla! 3.4.2, 3.4.3, 3.4.4, 3.4.5, and the staging branch
With a bitnami stack
PHP Version 5.4.39
Database Version 5.5.42

I was unable to replicate this issue. Either it was a fluke or we are missing something in the steps that caused the issue.