Can you exit and re-enter with the same ticket in Paris Metro?

According to this official transports of Paris web site:

The standard ticket (ticket t+) as purchased at the machines (i.e. not on board a bus from the driver) allows you to

• change metro lines as long as you do not leave the metro system (i.e. no re-entering)
• connect from metro to RER or RER to RER within Paris (you have to re-validate your ticket and need your ticket for the exit turnstiles of RER)
• connect bus to bus, bus to/from tram and tram/tram within 90 minutes from the first validation (no connections permitted if you buy a bus ticket from the driver, no connection to a bus/tram of the same line, i.e. no interruptions and no returns)

In the metro the ticket is checked at entrance turnstiles but keep your ticket until the end of your journey in case you run into a control. There is a time-limit of 2h for metro and RER (source in French) but it would be unusual to have a journey this long.

A ticket that allows more transfers is at least a day ticket (or longer durations).

The “ticket t+” (more detailed rules in French) is the only “general purpose” ticket in Paris (there are also concessionary tickets, point-to-point tickets for travel in the suburbs, etc., and passes for a day or more). You can buy it from a machine in any metro station or from various shops around the city and the suburbs. Buy them ten at a time (“un carnet” [œ?.ka?.n?]), it's significantly cheaper, there's no expiration date (though if you keep them for years beware that the tickets are magnetic and I think the rules changed recently so that you are no longer automatically due a replacement if they become demagnetized). Once you've bought the tickets, they're just 10 pieces of paper, not a single 10-use ticket, so multiple travelers can use one ticket each.

You can use each ticket for either a train trip or a bus/tram trip.

• If you pick a train trip, you can make one journey inside the network.

  • The basic idea is that you validate your tickets at fare gates when entering the underground network, and then the ticket serves for your whole trip.
  • There's a 2-hour time limit, this shouldn't be a concern in practice.
  • You can go anywhere in the metro network (both within and outside the Paris city limits), and you can use the RER inside the Paris city limits only (indicated as zone 1 on maps). In particular, to go to La Défense (which is outside the Paris city limits), you can use a ticket t+ on the metro but you need a different point-to-point ticket on the RER.
  • There are gates where your ticket is checked when switching between the metro to the RER and when exiting the RER. There are no ticket checks when exiting the metro, but there are generally doors and signs stating “your ticket is no longer valid beyond this point”.
  • A few RER/metro interchanges behave differently. Several stations on line C have no tunnel connecting them to the metro, but you are allowed to change between the RER and the metro by exiting the network, crossing the street (or even the river) and entering the network again at the corresponding station. These interchanges are indicated with dotted lines on the map.

• If you pick a bus/tram trip, you can make connections under certain conditions:

  • You need to validate your ticket each time you board a new vehicle.
  • You have 90 minutes from the first validation to the last validation (once you're on board that last bus, there's no time limit).
  • Within these 90 minutes, you can change as often as you like, but you are not allowed to backtrack (for a return trip, even within 90 minutes, you must use two tickets). You are also not allowed to get off and back onto the same line (so you can't really break your journey).
  • All lines in Paris and in the suburbs (RATP and Optile buses) are included except a few express lines (in particular the Orlybus and Roissybus airport lines).
  • For the night buses and a few long lines in the outer suburbs, you need to validate multiple tickets depending on the number of sections you travel.

• You can also use a ticket t+ for the Montmartre funicular. The same ticket cannot be used for anything else.

There are also passes:

• Daily passes, called Mobilis. The price for a pass that covers Paris is about that of 5 tickets, so it isn't worth it if you're staying in Paris. It can be worth it if you make extensive travels in the suburbs with train/bus connections (but note that the pass is more expensive if it covers more suburban zones).
• Tourist passes for up to 5 days, called Paris Visite. These are mostly tourist traps. Even with the (very minor) discounts at some tourist attractions, it's almost always less expensive to use ticket t+ and the occasional point-to-point ticket unless you travel in the suburbs a lot.
• Commuter passes. These are relatively cheap but valid only for a calendar week (Monday to Sunday), a calendar month, or a whole year. There's a one-time 5€ fee to buy the pass itself if you aren't a resident, but you can get it on the spot. Even with that fee, 26.25€ gets you a one-week pass (including the Orlybus and Roissybus airport buses and the train to Roissy, but not private coaches (e.g. Air France) or Orlyval (Orlybus is cheaper and often quicker anyway)), which can be cheaper than Paris Visite for a short stay if it's within a calendar week. Depending on how much you travel in the suburbs (most tourist attractions are in the city) and whether you arrive by plane, this may or may not be cheaper than using a ticket for each trip.

If you need enter/exit the subway multiple times, the most certain way is to buy a 24h unlimited ticket. It's more expensive of course, but if you travel a lot in one day it might be worth it.

Concerning the single fare ticket (which lasts 90 minutes and not two hours BTW), this site warns you that re-entry will not be possible:

Paris Metro Ticket transfers are allowed between: ... One Metro line to another, without exiting the confines of a station

Actually you can re-enter (metro) as long as it's in the time period. The gate will pass you and controller can only check time. I've just did it and had control - almost a heart attack :D