ruby, using regex to find something in between two strings

Question

Using Ruby + regex, given:

starting-middle+31313131313@mysite.com

I want to obtain just: 31313131313

ie, what is between

Answer 1

Here is a solution without regex (much easier for me to read):

i = to.index("+")
j = to.index("@")
to[i+1..j-1]