I've heard a lot of people pronounce it like, 'dashalb' but when I looked online for the pronunciation you can hear, 'deshalb'. Is this a regional thing?
The correct pronunciation is deshalb [ˈdɛsˈhalp], but you may hear it in many slight variations of the first vowel and the final consonant, e.g. [ˈdɪsˈhalp]. To me, dashalb doesn’t sound correct, but it may very well be a regional thing.