Non-ASCII/UTF-8 double quote characters in text format?

Hey, could use some help. I'm writing stuff in LibreOffice Writer (5.4.2.2) and it will sometimes put non-ASCII or UTF-8 " in the text. This causes an issue if I'm trying to use a regular expression to match the quotes as they are a non-standard character.

How can I force Libre Office to only use ASCII or UTF-8? How can I convert what is in whatever format to ASCII or UTF-8?

Thanks!

Leam

You do not say which Language or operating system or version of LibO you are
using, however in general terms I hope the explanation below helps.

LibreOffice only uses Unicode, that is to say UTF-8 in your terms. The
characters you are seeing are actually Unicode. This provides support for
about 108,000 characters. US-ASCII uses the first 127 of these. So, if you
go to INSERT > SPECIAL CHARACTERS the first (decimal) 127 or (hexadecimal)
U+7F characters are the same. The double left and right quotation marks are
down in about U+201C. The double quotation is (decimal) 34 U+22 is also
available in US-ASCII.

If your particular problem is DOUBLE QUOTATION Marks, the variation is
Country / Language dependant. But you will find assistance using the HELP >
QUOTATION MARKS. You need to turn off the AUTOCORRECT OPTION if it does not
do what you want. I quote from the HELP PANEL.

To Stop Replacing Quotation Marks
    1. Choose Tools - AutoCorrect - AutoCorrect Options.
    2. Click the Localised Options tab
    3. Unmark the "Replace" check box(es).