Use C# regex to convert casing in a string

Question

how can I convert this string:

bKk_035A_paint-House_V003
to
BKK_035a_paint-House_v003

with a regular expression (e.g. Regex.R

Answer 1

Final code I used

string input = "lAl_058a_068b_COMP_dsj_V001.txt";
string regex = @"^(?[a-z][a-z0-9]{1,2})_(?\d{3}[a-z]{0,2})_(?[a-z-]+)_(?v\d{3,5})$";

StringBuilder sb = new StringBuilder(input);
Match m = Regex.Match(input, regex, RegexOptions.IgnoreCase | RegexOptions.Multiline);
if (m.Success)
{
    Group g;
    g = m.Groups["Group1"];
    sb.Replace(g.Value, g.Value.ToUpper(), g.Index, g.Length);
    g = m.Groups["Group2"];
    sb.Replace(g.Value, g.Value.ToLower(), g.Index, g.Length);
    g = m.Groups["Group4"];
    sb.Replace(g.Value, g.Value.ToLower(), g.Index, g.Length);
}

Thanks for all your comments, Patrick