PHP preg_replace在null字符串上

Question

I am trying to insert a string of character or insert a word where $string value = Null; using PHP's preg_replace.
I couldn't find a proper regexp which will match null string.

my code is like this:

$string ={member_repeat_designation___designation};
$pattern ='//' ;
$replacement = 'Member';
return preg_replace($pattern, $replacement, $string);

Ok I am submitting the full code here why I need it.

$string1= "Ln. {member___first_name} {member___last_name<br>
{member_repeat_designation___designation}";
$string2="{member_repeat_designation___designation}";

$patterns=array();
$patterns[0]= '/President/';
$patterns[1]= '/Team Leader/';

$replacements=array();
$replacements[0]= '{member_repeat_designation___designation} {member___club_name}';
$replacements[1]= 'TL';

$patt=0;
$rep='Member';

$a=preg_replace($patterns, $replacements, $string1);
$b=preg_replace($patt, $rep, $string2);
return $a <br> $b;

Answer 1

Why do you need regex for this? PHP has an in-built function for this purpose:

$string = '{member_repeat_designation___designation}';
if( is_null($string) ) {

//string is null, do whatever

}
if( trim($string) == ''  ) {

//string is empty

}

Documentation: is_null()