The forward slash in regex is call delimiter, and it should be used to wrap around a pattern. I have seen people do it without it in MSL without a problem, but it can result an error if you don't include it for php.
Omitting the delimiter in MSL is fine - as long as
the the expression you aim for does not start with the "m" char. A leading "m" without delimiter defines the following char as the delimiter. But as long as you want to use the default delimiter "/", you don't have to put that "m".
$regex(bla,test) == $regex(bla,/test/) == $regex(bla,m/test/) == $regex(bla,m@test@)
$regex(bla,mudbug) == $regex(bla,/db/g)
Imo to always include the delimiter is good scripting habit.