In Islam a man can revoke the divorce he gives to his wife within the period of 'idah'* . After the expiry of the period of 'idah, the divorce is finalised and the husband and wife stand separated as a man and a woman, after which they can marry again if they so desire.
The number of revocations allowed during a marriage is two at most.** In other words, the husband can divorce his wife and then revoke the divorce within the period of 'idah'. If at some other time, he divorces his wife, he can revoke the divorce again within the period of 'idah. However, after this second revocation, there cannot be any revocation if the husband divorces his wife for the third time. In this case, the husband and wife are permanently separated as a man and a woman after the expiry of the period of 'idah. However, the Quran (2:230) has given one allowance even in case of this final revocation. The allowance is dependent on a contingent event: in case the woman is divorced after marriage to another man, her ex-husband can marry her if she so desires. The reason for this 'hinderance' is that marriage should not become something to be taken lightly.
If a man and woman marry merely for affording the woman the convenience of re-marrying her ex-husband, the divorce that is given by the man is called Halalah, which is prohibited in Islam, as is obvious from the Quranic verse referred to earlier:
And if he divorces her [after the first two revocations], then she is not lawful unto him thereafter until she marries another husband. Then, if the other husband divorces her, it is no sin for both of them that they re-unite, provided they feel that they can keep the limits ordained by Allah. These are the limits of Allah, which He makes plain for people.... (2:230)
In this verse, the verb tankiHa (until 'she marries') has been used. nikaH is a well-known term in the divine law and refers to marriage, which is a contract made by a man and woman who intend to live their lives together as husband and wife. Where the intention is to do that for a specific period of time, the appropriate word for such a contract seems to be prostitution, not nikaH, for such a 'marriage' is not nikaH -- it is a subterfuge. Moreover, it is also obvious from the words 'Then, if the other husband divorces her' that the divorce must not have been planned at the beginning of the marriage. The particle of condition, in, used here in fa'in talaqaha (Then, if he divorces her) is used for exceptional and contingent situations, that is such situations as are rare or happen by chance. Writes Islahi about the usage of in:
In Arabic in is not used for routine and usual matters. Usually, it is used for rare and contingent occurences. For regular matters, idha is used in Arabic. (tadabur-e-quraan, vol.1 p.638)
It must also be mentioned here that it is obvious from the Hadiith (saying of the Prophet sws) usually referred to as Hadiith-e-`usailah (bukhaarii, kitaabullibaas) that even if the contract of nikaH is valid, merely the desire on the part of the woman to return to her ex-husband is not a valid basis for the court to grant her divorce.