It would be a lot more helpful if you would supply a single snippet that produced the error, rather than lots of little bits with no context.

I haven't been able to reproduce your error exactly, but at least part of the problem is that \b means something different when interpolated in a string, than when interpolated in a regex. A backspace versus word boundary.

However, the fact that when you print $regex out having interpolated it into a string, it gets printed as \b\QRenata\E\b means that the contents of $regex must originally be (something like):

\\b\\QRenata\\E\\b
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And when you try to use that as a regex, the escaped (doubled) backslashes means that the regex engine will not recognise the escape sequences.

That I cannot reproduce the errors you are seeing probably means that your snippets do not reflect what you are really doing in your code, and I haven't been able to read between the lines sufficiently to guess what it is that you are actually doing.