regex - How to use ruby regexp to substitute string with a "callback function"-like manipulation

Question

I have the samples lines, and I want to substitute the leading #s with =s. (The first two lines) But instring.gsub!(/^#+w/, ""), I can not get the number of # which I want to substitute.

In javascript, I could use a callback function with the replace method, but how could I archive this Ruby?

##Command-line Tool
###Installment
This is a '#'.

The expected result:

==Command-line Tool
===Installment
This is a '#'.

Answer 1

a callback block function to the gsub method, probably. I am not sure what you had in mind but could be something like

s.gsub(/^(#+)w+/) {|m| m.gsub("#", "=") }