using System;
using System.Linq;
using System.IO;
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
var myLetter = "y";
var str = " modif-i-ed a-im y-i-eld";
var res = Regex.Replace(str, @"(\B[a-z](?=-))|-?[a-z]-",
m => m.Groups[1].Success ? m.Groups[1].Value : myLetter);
Console.WriteLine(res);
}
}
dXNpbmcgU3lzdGVtOwp1c2luZyBTeXN0ZW0uTGlucTsKdXNpbmcgU3lzdGVtLklPOwp1c2luZyBTeXN0ZW0uVGV4dC5SZWd1bGFyRXhwcmVzc2lvbnM7CgpwdWJsaWMgY2xhc3MgVGVzdAp7CglwdWJsaWMgc3RhdGljIHZvaWQgTWFpbigpCgl7CgkJdmFyIG15TGV0dGVyID0gInkiOwoJCXZhciBzdHIgPSAiIG1vZGlmLWktZWQgIGEtaW0gIHktaS1lbGQiOwoJCXZhciByZXMgPSBSZWdleC5SZXBsYWNlKHN0ciwgQCIoXEJbYS16XSg/PS0pKXwtP1thLXpdLSIsCgkJICAgICAgbSA9PiBtLkdyb3Vwc1sxXS5TdWNjZXNzID8gbS5Hcm91cHNbMV0uVmFsdWUgOiBteUxldHRlcik7CgkJQ29uc29sZS5Xcml0ZUxpbmUocmVzKTsKCX0KfQ==