正则表达式允许字母数字，只有一个空格，然后是字母数字

I've searched and searched and tried many different ways, but I can't seem to figure this out. I'm looking for a way to only allow alphanumeric characters, then only one space, then alphanumeric characters. I'm sure it's easy, but I don't know it.I've searched and searched and tried many diffe