Test if a string is equal to one of the strings (with regex)

Question

How could this be done with regex?

 return ( $s=='aa' || $s=='bb' || $s=='cc' || $s=='dd' ) ? 1 : 0;

I am trying:

 $s = 'aa';
 $result = preg_match( '/(aa|bb|cc|dd)/', $s );
 echo $result; // 1 

but obviously this returns 1 if $s contains one or more of the specified strings (not when it is equal to one of them).

Answer 1

You need to use start ^ and end $ anchors to do an exact string match.

$result = preg_match( '/^(aa|bb|cc|dd)$/', $s );