This makes any filter that starts and ends in forward slashes act as a
regex filter instead of a simple substring filter.
In case any existing plain-text rule already matches this
it will need to be updated adding an additional layer of slashes
and escapes as needed. However, this is thought to be sufficiently uncommon.
Instead of using trailing flags as modifier complicating parsing further,
any modifications to regex matching must be done via local modifiers. See
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Modifier
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