I want to search and replace each occurrence of a certain pattern with a decimal number which begins at `1` and increments by one for each match.

I can find similarly worded questions that turn out not to be about incrementing a counter but modifying each match by a fixed amount. Other similar questions are about inserting line numbers rather than an incrementing counter.

Example, before:

    #1
    #1.25
    #1.5
    #2

After:

    #1
    #2
    #3
    #4

My real data has a lot more text all around the stuff I want to re-number.