Doing an image search for "handwritten algebra" I find quite a few examples of ")(". "I was taught the ")(" way at primary school" is a perfectly reasonable re

Doing an image search for "handwritten algebra" I find quite a few examples of ")(". "I was taught the ")(" way at primary school" is a perfectly reasonable response to your question, which wondered whether it was just one person's preference. That answer tells you it isn't. Nowhere did the reply suggest it wasn't regional, nor, for that matter, that "/ \" isn't regional. Both are presumably regional, but whether one is more common requires a fair bit of work to answer.