Try the following regex:
Format\(([^,]*),\s*([^\)]*)\)
and the following replacement string:
$1.ToString($2.Value);
in C#:
var input = "Format(Element[Be], Element[Be])";
var result = Regex.Replace(input, @"Format\(([^,]*),\s*([^\)]*)\)", "$1.ToString($2.Value)");