The question was whether “every developer should know ___”. Notice that all of us said no on RIA platforms like Silverlight, despite the fact that some of us are really into Silverlight.

Also - why would you assume that we don’t know regular expressions? That’s a pretty condescending assumption. We’ve put the time into learning regular expressions, see that they’re a useful tool, and didn’t think that they made the cut for required, basic knowledge that every single web developer should know. Would every developer benefit from knowing assembly language? Sure! Is assembly language core, basic knowledge that every developer should put time into learning? Not in my book.

I’d rather web developers put that time into learning the HTML/CSS platform better. I’ve worked with a lot of web developers who were handy with a regex but couldn’t write cross-browser code or table-less layouts. That’s a mistake.

I do think people overrate the utility of regex’s. Having used regular expressions extensively in a custom CMS application, my personal experience is that they’re easy enough when you’re writing the application, but the maintenance and edge cases kill you. So in my opinion, they’re fine for things like e-mail validation, but pretty error prone for things like HTML manipulation (c.f. all the security vulnerabilities Jeff Atwood hit with StackOverflow’s HTML manipulation).

The above posted information explain the difference in the two disciplines of a web designer and a web developer. If you have knowledge at design and develop side no need to hire.Really it helps to me a lot when i am going to hire designers and developers.