Question

$string = "   Some string  ";
//the output should look like this
$output = "___Some string__";

So each leading and trailing whitespace replaced by underscore.

I found the regex for this in C here: Replace only leading and trailing whitespace with underscore using regex in c#
but i couldn’t make it work in php.

Answer 1

You could use a replace like:

$output = preg_replace('/\G\s|\s(?=\s*$)/', '_', $string);

\G matches at the beginning of the string or at the end of the previous match, (?=\s*$) matches if the following is only whitespace at the end of the string.
So this expression matches each of the spaces and replaces them with a _.