You have to be careful. If you try to use {2} in an expression, VC will try to replace it with payload 2. If there is no payload 2 then it will leave it as is, so if you know your command is being executed without any payloads then you are free to use {2} in your expression.
If you know your command has a payload 2 attached to it, then you will need to avoid using the regex expression {2}. This is relatively easy to do.
For example if you want to match exactly two digits using regex you can use any of the following patterns
\d\d
\d{2}
\d{2,2}
So you can use the first or the last option and just not use the middle one.