It is easier to make the "j" said for "rejex". The "g" sound is more effortful (it is "made" more in the back of the mouth, compared to "j"). Laziness wins the day again on pronunciation xD

I would pronounce it “regular expression.” I can see shortening it in text, it is a lot of letters, but reg-eks seems only marginally shorter than “regular expression” and the latter has normal, easy flowing words without weird stop in the middle. “Reglr spression” “Regula espressen.”