[rgl]

var s = "str0\nstr1\n/*\nstr2\nstr3\nstr4\n*/\nstr5\nstr6";

s = s.replace( /\/\*[\S\s]*\*\//g, function( ss ) {
  return ss.replace( /(.+)/g, "[c]$1[/c]" );
} );

alert( s );