[[HiC]TheKiller]

pawn Code:

stock str_replace(currentstr[], replacestr[], findstr[])
{
    while(strfind(currentstr, findstr) != -1)
    {
        new pos = strfind(currentstr, findstr), len = strlen(replacestr);
        strdel(currentstr, pos, pos+len);
        strins(currentstr, replacestr, pos);
    }
    return 1;
}
public OnPlayerText(playerid, text[])
{
    str_replace(text, "$1234", "$$$");
    return 1;
}

Give that a go. Sorry it's a bit inefficient, I made it really fast.