gistfile1.js

/*
html:
<sg:wrapper>
<div class="wrapper" style="background: red; padding: 0.5em;">
  <sg-wrapper-content/>
</div>
</sg:wrapper>
*/

var re = /<sg\:wrapper>([\s\S]*?)<\/sg\:wrapper>/g,
  match = html.match(re);

console.log(match);

/* Output:

[ '<sg:wrapper>\n<div class="wrapper" style="background: red; padding: 0.5em;">\n  <sg-wrapper-content/>\n</div>\n</sg:wrapper>' ]

*/