The Act of Contrition is best understood as a genre. No single wording is canonical, and the church has approved many forms, from the terse Latin Deus meus, ex toto corde paenitet me omnium meorum peccatorum to vernacular versions that vary by country and by generation. The revised Rite of Penance issued after the Second Vatican Council makes the plurality official, printing a selection of prayers of the penitent among which the traditional act stands beside newly composed forms and biblical texts, including the tax collector's plea, O God, be merciful to me, a sinner, so that a penitent may express contrition in scripture's own words.
The prayer's uses reach beyond the confessional. Catholic practice commends an act of contrition in night prayer, after an examination of conscience, so that no day closes with sorrow unspoken, and in danger of death the act carries particular weight, since perfect contrition, sorrow from love of God with the intention of confessing, is held to reconcile even when no priest can be reached. Chaplains and prayer books of every war have therefore carried it, and teaching the dying to make an act of contrition has been part of Catholic deathbed ministry for centuries, the genre's humblest and most serious employment.