/([A-Za-z]+\(\))
/g
They are delimiters - to delimit your string from the modifiers/switches like "g" for global, "S" for strip, or "i" for case-insensitive.
You need to know that if an expression starts with the "m" char, the next char will be used as the delimiter. The "/" char is the "default" delimiter, and does not require the leading "m". Any char can be used as a delimiter: "m/test/g" is the same as "/test/g" or "m@test@g". And if you want to match the delimiter literally, you have to escape it of course.
Delimiters aren't required all the time, but it's a good habit to always put them:
While $regex(go test me,test) will do the same as $regex(go test me,/test/), $regex(surround,mono)
will find a match - to some people's surprise - as it uses the "o"'s of "mono" as delimiter, thus matches like $regex(surround,m/n/) - which is the same as $regex(surround,n).
Here is more info about it.
