$strstr usage with combination literal & placeholders

Value in tag CONTENTGROUP is "Energy 4"

I am trying to get $strstr to evaluate to something other than 0.
Using a Format Value action, my script is:
$strstr(%contentgroup%,Energy\s\d+)

Why doesn't the script evaluate to something greater than 0?

Because it is fairly like that you do not have the string "backslash-s-backslash-d-plus" in the field contentgroup.
$strstr() only compares string constants or field variables that result in a string constant and no patterns.

See also the help on string functions: