[Basicz]

Another version ( loops ), tested with Slice's benchmark code, it's faster.
( Also removed invalid character checks for the test on both function; nor the oneCharAllowed ).

pawn Код:

stock getPlayerFirstName( playerid )
{
    new
        playerName[ 24 ]
    ;

    GetPlayerName( playerid, playerName, 24 );

    for ( new i, nameLen = strlen( playerName ); i != nameLen; ++ i )
    {
        if ( playerName[ i ] == '_' && i > 0 )
        {
            playerName[ i ] = EOS;
        }
    }

    return playerName;
}

And thanks for the help!