private function getExpression()
    {
        if ($this->expression === null) {
            $this->expression =
                '/(<\?xml.*\?>)?([ \t\n\r]*)(<' .
                preg_quote($this->documentElement, '/') .
                '.*xmlns="' .
                preg_quote($this->namespace, '/') .
                '".*>)/is';
        }

        return $this->expression;
    }
private function getExpression()
    {
        if ($this->expression === null) {
            $this->expression =
                '/(<\?xml.*\?>)?([ \t\n\r]*)(<' .
                preg_quote($this->documentElement, '/') .
                '.*xmlns="' .
                preg_quote($this->namespace, '/') .
                '".*>)/is';
        }

        return $this->expression;
    }
private function getExpression ( ) { if ( $ this -> expression === null ) { $ this -> expression = '/(<\?xml.*\?>)?([ \t\n\r]*)(<' . preg_quote ( $ this -> documentElement , '/' ) . '.*xmlns="' . preg_quote ( $ this -> namespace , '/' ) . '".*>)/is' ; } return $ this -> expression ; }
Computes and returns regular expression pattern for matching and replacement.

@return string
Computes and returns regular expression pattern for matching and replacement .