<?php
$yourstring = "{{SOME_STRING}}
{{    SOME_STRING}}
{{  SOME_STRING   }}
{{  SOME-STRING      }}
{{  SomE STRING  }}";
$regex = '/{{\s*\K[^}]+?(?=\s*}})/';
preg_match_all($regex, $yourstring, $matches);
print_r($matches[0]);