The Desired Results

Suppose that we want a regex to match a sum of two variables.

x + y
 
length + width                    // no underscores in this example 
 
rectan_width + rectan_height    // spaces around `+`

rectan_width+rectan_height      // no spaces around `+`

Attempt at a Functioning Regular Expression

Here is what I have so far…

(?<=s|^|b)(?:[_/]b|b[_/]|d*.?d+|[A-Za-z0-9]|([A-Za-z0-9]+))+(?=s|$|b) *+ *(?<=s|^|b)(?:[_/]b|b[_/]|d*.?d+|[A-Za-z0-9]|([A-Za-z0-9]+))+(?=s|$|b)

The demonstration can be viewed here.