I’ll tell you right off that “with regard to” is the correct attributive phrase, and that “with regards to” is nonstandard usage—an expression that you shouldn’t ever catch your tongue saying or ever catch yourself putting down in writing. I’m saying this as forcefully as I can because I think it’s a major embarrassment that so many otherwise English-savvy people acquire the habit of using “with regards to” blissfully unaware that it is bad English.
I discussed the difference between those two “regard” expressions in an essay, “With regard to ‘with regards to’,” that I wrote for my English-usage column in
The Manila Times way back in 2007. I posted that essay in the Forum in 2011 under the heading
“How to deal with superiors who use the ‘regard’ idioms wrongly,” and you can read it right now by clicking the indicated link. I’m confident that when you’re done, your confusion will be cleared out for good and you’d be able to banish “with regards to” and its likewise flawed variant “in regards to” from both your written and spoken English forever.