Replace each tab ONLY at the beginning of each line with spaces

Question

So replacing all tabs in a file with spaces is not hard.
In vim for example, I can do %s/\t/ /gc

And if I want to replace the ones at the beginning of each line, not the one in the middle I can do %s/^\t/ /gc

But if there are line with one and lines with more tabs at the beginning, and lines with tabs in the middle, and I want to replace each tab in the beginning of a line with spaces to keep the indentation structure of the file, that is what I don't know how to do.

In vim or sed or generally using regular expressions.

Answer 1

You can use the evaluation register to replace any number of tabs with the appropriate number of spaces. For example:

:s/^\t\+/\=repeat('    ',len(submatch(0)))

Explanation:

:s/                                         " Replace
   ^                                        " At the start of a line
    \t\+                                    " One or more tabs
        /\=                                 " With the following evaluated as vimscript:
           repeat('    ',len(submatch(0)))  " 4 spaces times the length of the previously matched string