Exec Transformation in Item

I am not sure what you want to do is possible. Maybe an example would help. Where does the bit value come from? How is it stored? Etc. Maybe we could suggest a solution.
As your question raises the need of more information I will post some information.

What i know of the regex-transformation-service is that:

I only used exec-retransformation to set the output argument to the command line to execute. Have a look in the example.
https://community.openhab.org/t/1-openhab-433mhz-radio-transmitter-tutorial-exec-binding-explained-in-detail/34977/20

Using transformation in the exec thing for example has the drawback that the result will always be a string.

You may use a rule to make the transformation as shown here.

Maybe with this information you could solve your problem or formulate example or question which explains what you try to do explicitly.