$string = preg_replace('/W+$/', '', $string); // converts `#str|ng#` to `#str|ng`

$string = preg_replace('/^W+/', '', $string); // converts `#str|ng#` to `str|ng#`

$string = preg_replace('/^W+|W+$/', '', $string);

$string = preg_replace('/^[W_]+|[W_]+$/', '', $string);

$string = trim($string, "#");