A backslash followed by a character not listed below causes an error message. The character with the value given by ooo , which is 1 to 3 octal digits. Although GNU tr does not support the System V syntax that uses square brackets to enclose ranges, translations specified in that format will still work as long as the brackets in string1 correspond to identical brackets in string2. If n begins with a 0, it is interpreted in octal, otherwise in decimal. Doing this specifies case conversion.
Equivalence classes are a recent invention intended to support non-English alphabets.
But there seems to be no standard way to define them or determine their contents. Characters not in set1 are passed through unchanged. When a character appears more than once in set1 and the corresponding characters in set2 are not all the same, only the final one is used. A common use of tr is to convert lowercase characters to uppercase. This can be done in many ways.