Right answer to
Can you exit and re-enter with the same ticket in Paris Metro?
is : no for a basic subway ticket and yes your ticket is valid for 1 hour and 30 minutes for buses and 2 hours for train, while you stay inside the public transportation network.
Actually it is a single trip ticket which mean you can use it from point A to point B. To do this trip you may need to take multiple subway lines or bus lines and you will not have to pay to take those different lines.
Warning ! With the same basic single-trip ticket you cannot connect between underground (metro or RER) and above ground (bus and tram)
@mts is talking about a website which is the official one about Parisian public transportation network.
Current tickets available here, current prices are :
Ticket t+ (Basic Subway Ticket)
Paris Visite with discount and all prices about suburbs and other area next to Paris.
I have lived in Paris for 20 years so if you need more information about that it would be a pleasure.