some text <p id='item_1' class='item'>multiline contentrnrn for <br/>remove</p><br clear='all' id='end_of_item_1'/><p id='item_2' class='item'>another multiline contentrnrn</p><br clear='all' id='end_of_item_2'/>

<p id='item_1' class='item'>multiline contentrnrn for <br/>remove</p><br clear='all' id='end_of_item_1'/>

var id = 'item_1';
var patt=new RegExp("<p id='"+id+"'(.)*|([Ss]*?)end_of_"+id+"'/>","g");
var str="some text <p id='item_1' class='item'>multiline contentrnrn for <br/>remove</p><br clear='all' id='end_of_item_1'/><p id='item_2' class='item'>another multiline contentrnrn</p><br clear='all' id='end_of_item_2'/>";
document.write(str.replace(patt,""));

some text for
<br>
remove
<p></p>
<br id="<p id=" class="item" clear="all" item_2'="">
another multiline content
<p></p>
<br id="end_of_item_2" clear="all">

var s        = "some text <p id='item_1' class='item'>multiline contentrnrn for <br/>remove</p><br clear='all' id='end_of_item_1'/><p id='item_2' class='item'>another multiline contentrnrn</p><br clear='all' id='end_of_item_2'/><ul><li>";
var id       = 'item_1';

var patt     = new RegExp ("<p[^<>]*\sid=['"]" + id + "['"](?:.|\n|\r)*<br[^<>]*\sid=['"]end_of_" + id + "['"][^<>]*>", "ig")

var stripped = s.replace (patt, "");

"some text <p id='item_2' class='item'>another multiline content

</p><br clear='all' id='end_of_item_2'/><ul><li>"

var item1 = document.getElementById('item_1'),
    endOfItem1 = document.getElementById('end_of_item_1');

item1.parentNode.removeChild(item1);
endOfItem1.parentNode.removeChild(endOfItem1);

<ps+id='([a-zA-Z0-9_]+)'.*?id='end_of_1's*/>

var patt=new RegExp("<ps+id='"+id+"'.*?id='end_of_"+id+"'s*/>","g");