Any idea why my regex failed?

What is the type of the sfdcEmailOptOut attribute in Braze? Is it a string, because that's the only type that would make sense to use a regex on... but if you're only testing for a single value, then just use matches rather than a regex.

Or, ideally, get the type of the attribute to be boolean and re-import.

Or... have you changed the type of the attribute recently? There can be cases where you have data loaded as one type, then you change the type and the UI can get a bit confused and offer you filters as if it was a string, but then not match.

As a side-note, having attributes and segments indicating a positive condition (is Opted In) can be easier to manage and communicate. Rather than all the stacked double negatives - but you might not have that as an option right now.