PHP regex replace white space by if it is following single letter

Question

I have user defined string (html formated string to be saved and used in web) and need to find a way to replace each white space which is right after a single letter by

Answer 1

without regex

$str = "this is a string" ;
$s = explode(" ",$str);
foreach ($s as $i => $j){
    if (strlen($j)==1){
        $s[$i]="$j ";
    }
}
print_r ( implode(" ",$s) );