string[] arr = System.Text.RegularExpressions.Regex.Split("aa123aa345bb","aa");

------解决方案--------------------

C# code
private string[] Test(string inpstr, string bstr, string estr)
{
string pstr = @"(?<="+bstr+").*?(?="+estr+")";
MatchCollection mc = Regex.Matchs(inpstr,pstr);//手打，可能有错
string[] rstrs = new string[mc.Count];
for(int i=0;i<mc.Count;i++)
{
rstrs[i] = mc[i].Value;
}
return rstrs;
}

------解决方案--------------------
正则