Q:

What is the syntax of a obligatory regex element? 

How can I make the parameter after : obligatory?  I've tried a couple of online web tools but they couldn't help me. 
^(?<first>.*?):(?<second>.*?)$

A:

I guess what you mean is, you want that part to contain at least one character?  In that case, use .+?  instead of .*?. (*  means "zero or more", + means "one or more".)  Better yet, write:
^(?<first>[^:]*):(?<second>.+)$

