This regexp is saying extract and return the numeric text that is surrounded by 
any text ___ NUMERIC ___  any text
If the Current value =  "abc ___ 1234 ___ abc" the result would be 1234.

.+\s___\s(\d+)\s___\s.+
any text, a space, 3 underscores, a space, a number, a space, 3 underscores, a space and then text

If you matched the following text your results would be:
abc ___ 1234 ___ abc  = 1234
abc ___ 1234 ___   No match since there is no text following the underscores
abc ___ 1234 ___ abc ___ 5678 ___ abc = 5678   ( it would match the last valid number )
abc ___ 1234 ___ abc ___ 5678 ___  = 1234 ( there is no text after the 5678 ___ )

What exactly are you trying to do?
Andrew

The link you provided does not exist