How do I delete folders using regex from Linux terminal

Use find to filter directories

$ find . -type d -name "img*" -exec rm -rf {} \;

As it was mentioned in a comments this is using shell globs not regexs. If you want regex

$ find . -type d -regex "\./img.*" -exec rm -rf {} \;

you could use

rm -r img*

that should delete all files and directories in the current working directory starting with img

EDIT:

to remove only directories in the current working directory starting with img

rm -r img*/