No. That option was wrongly and confusingly named in previous versions of TextPad: it merely changed the syntax to a more modern and more convenient style. What would you require of it?
What TextPad pre-7 called "Posix" style differed from the default style only in the way backslashes were used. In that respect "Posix" style is like the style used by TextPad 7. Most TexPad pre-7 "Posix"-style regexes will work in TextPad 7. Have you found one that doesn't?