Replace all characters in a regex match with the same character in Vim

As an alternative to using the :substitute command (the usage of which is already covered in @Peter’s answer), I can suggest automating the editing commands for performing the replacement by means of a self-referring macro.

A straightforward way of overwriting occurrences of the search pattern with a certain character by hand would the following sequence of Normal-mode commands.

1. Search for the start of the next occurrence.

   /\(hello\)\+
   

2. Select matching text till the end.

   v//e
   

3. Replace selected text.

   r-
   

4. Repeat from step 1.

Thus, to automate this routine, one can run the command

:let[@/,@s]=['\(hello\)\+',"//\rv//e\rr-@s"]

and execute the contents of that s register starting from the beginning of the buffer (or anther appropriate location) by

gg@s

%s/\v(hello)*/\=repeat('-',strlen(submatch(0)))/g