Reach and Attacks of Opportunity: How to treat an approaching enemy?

Question

Imagine this scenario:

Red has a reach weapon and he's threatening B1 and B2.

Green has a normal weapon and wants to get closer to Red in order to attack him. ( C2 )

So green takes a first step into the reach of Red's weapon, but provokes no attack of opportunity (not yet).

Green then takes a second step into C2, leaves the threatened area of red and arrives in C2.

This should normally provoke an attack of opportunity, but since Red is not threatening C1 and C2, I'm not sure if he gets the AoO.

Green is using no particular feat or acrobatics check to avoid the AoO.

Actually, I found no evidence in rules as written, so any reference is gladly welcome.

Answer 1

Green provokes an Attack of Opportunity.

d20PFSRD:

Provoking an Attack of Opportunity

Two kinds of actions can provoke attacks of opportunity: moving out of a threatened square and performing certain actions within a threatened square.

Moving

Moving out of a threatened square usually provokes attacks of opportunity from threatening opponents. There are two common methods of avoiding such an attack—the 5-foot step and the withdraw action.

The action that provokes the AoO isn't moving between threatened squares, but "moving out of a threatened square". In your scenario, green is in a threatened square and moves out of it, the AoO is resolved immediately while green is leaving the square, not after.

d20PFSRD:

An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).