string sStringReplace(string sName, string sOld, string sReplace)
{
    integer iStartPos;
    integer iEndPos;
    if (llSubStringIndex(sName, sOld) != -1)
    { //if a match was found
        while (llSubStringIndex(sName, sOld) != -1)
        { //repeat
            iStartPos = llSubStringIndex(sName, sOld);
            iEndPos = iStartPos + llStringLength(sOld);
            sName = llGetSubString(sName, 0, iStartPos-1) + sReplace + llGetSubString(sName, iEndPos, -1);
        }
        return sName;
    }
    else
    {
        return sName;
    }
}

default
{
    state_entry()
    {
        string sTest = "Martin,Sebastian,Hannes,Ferdinand";
        string sKomma = ",";
        string sSemikolon = ";";
        sTest = sStringReplace(sTest, sKomma, sSemikolon);
        llSay(0, sTest);
    }
}