String sub regex help

Hi! I'm trying to rename a batch of images named similar to the following:

• POS1112_SS26_PHONE_170X300MM.jpg
• POS1118_SS26_ARCH_170X300MM.jpg

It's a mix of JPGs and tiffs but they all follow the same layout as above. I only want to remove everything after the first underscore to leave them as-

• POS1112
• POS1118

I'm aware of regex but sadly don't have a clue where to start with it. 

Thanks in advance!