NEED DART HELP

I want to replace part of the ntiger number first number with 1 and next 4/5 numbers with 0. No changed in first number is 0.

For example,

1. I’m getting 8123456789 from backend, but I want to display it as 1000056789.
2. I’m getting 9665789456789 but I want to display it as 1000009456789.

Would like to take decision number of changing numbers(chracter) with a logic like if user start with A then would like to change first 6 numbers or if user start with B then would like to change first 5 numbers.

Please advise. Many thanks.