escape pattern for EV3P for perl/latex brace conflict (hotfix)

[Alex-Jordan]

This is an alternative to #915, as mentioned there. This approach escapes a brace that we are pretty sure is not intended to be part of a perl hash value accessor. I lean toward this approach since it doesn't require macro authors to know about the underlying issue with braces. And if it needs adjusting, it's just in one place.

This is a conservative regex pattern that only escapes the brace if followed by backslash,backslash,not-backslash. It could be a little less conservative. Or a lot less conservative like s/\{(?!\s*[\$\w])/\\\{/g, always escaping a brace unless it is followed by a word character or a dollar sign, which could indicate the brace was indeed meant to be part of a hash value accessor.

I'll wait for discussion to see if this goes forward ro needs changing, before opening a develop version.

[drgrice1]

I think you have not yet seen my comment at #914 (comment). As per that comment, this should be targeted for develop, and adjusted a bit.

[Alex-Jordan]

Closing in favor of #914, #915, and #917.