<textarea id="txt">
lkasdjflwqerqrjdfl;sa <start> asfdlk;jasdklfjas <end> adfasdflkj;kljasdf
asdfiojas;dfkjas;lkdf <start> alkjfasl;kjdfas <end> fsdafsadf asfdasdf asdfasdfasd <start> 1235456 <end> asdfasdfasdfsad
</textarea><script>
var txt=document.getElementById("txt").value;
var reg=/<start>(?:(?!<end>).)*<end>/g;
var arr=txt.match(reg);//得到的是数组
alert(arr.join(""));
</script>
如果标签也有换行的话!~得改改~不过一般标签是不允许换行的~~
<textarea id="txt">
lkasdjflwqerqrjdfl;sa <start> asfdlk;jasdklfjas <end> adfasdflkj;kljasdf
asdfiojas;dfkjas;lkdf <start> alkjfasl;kjdfas <end> fsdafsadf asfdasdf asdfasdfasd <start> 1235456 <end> asdfasdfasdfsad
</textarea><script>
var txt=document.getElementById("txt").value;
var reg=/<\n?s\n?t\n?a\n?r\n?t\n?>(?:(?!<\n?e\n?n\n?d\n?>)[\s\S])*<\n?e\n?n\n?d\n?>/ig;//标签可换行,而且不区分大小写
var arr=txt.match(reg);
alert(arr.join(""));
</script>
lkasdjflwqerqrjdfl;sa <start> asfdlk;jasdklfjas <end> adfasdflkj;kljasdf
asdfiojas;dfkjas;lkdf <start> alkjfasl;kjdfas <end> fsdafsadf asfdasdf asdfasdfasd <start> 1235456 <end> asdfasdfasdfsad
</textarea> <script>
var txt=document.getElementById("txt").value;
var reg=/ <\n?s\n?t\n?a\n?r\n?t\n?> (?:(?! <\n?e\n?n\n?d\n?> )[\s\S])* <\n?e\n?n\n?d\n?> /ig;//标签可换行,而且不区分大小写
var arr=txt.match(reg);
alert(arr.join(""));
</script>
lkasdjflwqerqrjdfl;sa <start> asfdlk;jasdklfjas <end> adfasdflkj;kljasdf
asdfiojas;dfkjas;lkdf <start> alkjfasl;kjdfas <end> fsdafsadf asfdasdf asdfasdfasd <start> 1235456 <end> asdfasdfasdfsad
</textarea><script>
var txt=document.getElementById("txt").value;
var reg=/<\n?s\n?t\n?a\n?r\n?t\n?>(?:(?!<\n?e\n?n\n?d\n?>)[\s\S])*<\n?e\n?n\n?d\n?>/ig;//标签可换行,而且不区分大小写
var arr=txt.match(reg);
alert(arr.join(""));
</script>