RegEx for adding a space in a special pattern - php

Question

Quick note: I know markdown parsers don't care about this issue. It's for the sake of visual consistency in the md file and also experimentation.
Sample:
# this
##that
###or this other
Goal: read each line and,if a markdown header does not have a space after the pound/hashtag sign, add one so that it would look like:
# this
## that
### or this other
My non-regex attempt:
function inelegantFunction (string $string){
$array = explode('#',$string);
$num = count($array);
$text = end($array);
return str_repeat('#', $num-1)." ".$text;
}
echo inelegantFunction("###or this other");
// returns ### or this other
This works, but it has no mechanism to match the unlikely case of seven '#'.
Regardless of efficacy, I would like to figure out how to do this with regex in php (and perhaps javascript if that matters).

Answer 1

Try to match (?m)^#++\K\S which matches lines starting with one or more number signs then replace it with $0 in your function:
return preg_replace('~(?m)^#++\K\S~', ' $0', $string);
See live demo here
To limit the number of #s to six use:
(?m)^(?!#{7})#++\K\S

Answer 2

I'm guessing that a simple expression with a right char-list boundary might be working here, maybe:
(#)([a-z])
If we might be having more chars, we can simply add it to [a-z].
Demo
Test
$re = '/(#)([a-z])/m';
$str = '#this
##that
###that
### or this other';
$subst = '$1 $2';
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;