In Texas, ex-offenders regain the right to vote only after completion of both their prison term and time served on probation and parole. It is recommended that ex-offenders re-register or update their voter registration after completion of sentence.
They also provide links to further information regrading such laws across the country from The Brennan Center and The Sentencing Project.
Looking at the map in the Brennan Center site made me wonder whether there is a relationship between state laws about voter and partisan results. One of the arguments made about party positions on the issue is that it all boils down to whether they think the ex-felons will vote for them or not. Here's an opportunity to find out if the argument has merit.