find(pattern, replacement) {
        pattern = regexify(pattern);

        let matches = [];
        this._rows.forEach(row => {
            if (!row) return;
            matches = matches.concat(row.find(pattern, replacement));
        });

        return matches;
    }
Find the given pattern in the sheet and optionally replace it.
@param {string|RegExp} pattern - The pattern to look for. Providing a string will result in a case-insensitive substring search. Use a RegExp for more sophisticated searches.
@param {string|function} [replacement] - The text to replace or a String.replace callback function. If pattern is a string, all occurrences of the pattern in each cell will be replaced.
@returns {Array.<Cell>} The matching cells.