正则表达式的[] +

Question

<script>var str="chrome/54.0.2840.71  safari/537.36 ";  var patt1=/chrome\/[\d.]+/;document.write(str.match(patt1));</script>这里的[],+但代表什么意思。var patt1=/chrome\/\d.+/;如果这样写会输出chrome/54.0.2840.71 safari/537.36，而加上[]就会只输出chrome/54.0.2840.71