var s1 = "abC...ABc..aBC....abc...ABC";
var s2 = s.replace(/some clever regex for abc/g,"#"+original abc match+"#");

"#abC#...#ABc#..#aBC#....#abc#...#ABC#"

var s1 = "abC...ABc..aBC....abc...ABC";

var s2 = s1.replace(/abc/ig,function (match) {
  return "#" + match + "#"  ;
}
 );

alert(s2);