All regex parameters that accept the optional surrounding slashes (e.g. match=/hello/) do not treat an empty string and an empty pattern the same way. For example, a REST-API request with "match": "//" and "match": "" are not the same: in the former case the pattern is treated as not empty whereas in the latter case it is treated as empty.
In practice this means that disabling a pattern is not possible with the syntax that uses the optional slashes and a literal empty string must be used. This is especially problematic in the cases where shell escaping makes it difficult to pass empty strings, for example with maxctrl alter filter.