Ultisnips - Variables into conditional replacements are not replaced

[z0rzi]

For example:

SnakeCase: ${1:SomeVariable}
camel_case:  ${1/^(.[a-z]*)([A-Z][a-z]*)$/\l$1(?2:-\l$2)/g}

Will give:

SnakeCase: SomeVariable
camel_case:  some-$2

The $2 is not replaced by variable.