SO fiddle

Test Fiddle mainly for SO Questions

by Nick Craver

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
a1-a3<br />
<input type="checkbox" id="a1" />
<input type="checkbox" id="a2" />
<input type="checkbox" id="a3" />
<br />
b1-b3<br />
<input type="checkbox" id="b1" />
<input type="checkbox" id="b2" />
<input type="checkbox" id="b3" />

JavaScript

var checks = document.getElementsByTagName('input');
for(var i = 0, cb; cb = checks[i]; i++) {
   if(cb.type.toLowerCase() == 'checkbox' && cb.id && cb.id.substring(0, 1) == 'a')
     cb.checked = true;
}