Here's my example. If I want to use a regex to replace tabs in the code with spaces, but wanted to preserve tab characters in the middle or end of a line of code, I would use this as my search string to capture each tab character at the start of a line: `^(\t)+`

Now, how could I write a search string that replaces each captured group with four spaces? I'm thinking there must be some way to do this with backreferences? 

I've found I can work around this by running similar regex-replacements (like `s/^\t/ /g`, `s/^ \t/ /g`, ...) multiple times until no more matches are found, but I wonder if there's a quicker way to do all the necessary replacements at once.

**Note:** I used `sed` format in my example, but I'm not sure if this is possible with `sed`. I'm wondering if `sed` supports this, and if not, is there a platform that does? (e.g., there's a Python/Java/bash extended regex lib that supports this.)