The format of the set1 and set2 arguments resembles the format of regular expressions; however, they are not regular expressions, only lists of characters. Most characters simply represent themselves in these strings, but the strings can contain the shorthands listed below, for convenience. Some of them can be used only in set1 or set2, as noted below.
The following backslash escape sequences are recognized:
The 8-bit character with the value given by ooo, which is 1 to 3 octal digits. Note that ‘\400’ is interpreted as the two-byte sequence, ‘\040’ ‘0’.
While a backslash followed by a character not listed above is interpreted as that character, the backslash also effectively removes any special significance, so it is useful to escape ‘[’, ‘]’, ‘*’, and ‘-’.
The notation ‘m-n’ expands to all of the characters from m through n, in ascending order. m should collate before n; if it doesn’t, an error results. As an example, ‘0-9’ is the same as ‘0123456789’.
tr does not support the System V syntax that uses square
brackets to enclose ranges. Translations specified in that format
sometimes work as expected, since the brackets are often transliterated
to themselves. However, they should be avoided because they sometimes
behave unexpectedly. For example, ‘tr -d '[0-9]'’ deletes brackets
as well as digits.
Many historically common and even accepted uses of ranges are not
portable. For example, on EBCDIC hosts using the ‘A-Z’
range will not do what most would expect because ‘A’ through ‘Z’
are not contiguous as they are in ASCII.
If you can rely on a POSIX compliant version of
the best way to work around this is to use character classes (see below).
Otherwise, it is most portable (and most ugly) to enumerate the members
of the ranges.
The notation ‘[c*n]’ in set2 expands to n copies of character c. Thus, ‘[y*6]’ is the same as ‘yyyyyy’. The notation ‘[c*]’ in string2 expands to as many copies of c as are needed to make set2 as long as set1. If n begins with ‘0’, it is interpreted in octal, otherwise in decimal.
The notation ‘[:class:]’ expands to all of the characters in
the (predefined) class class. The characters expand in no
particular order, except for the
which expand in ascending order. When the --delete (-d)
and --squeeze-repeats (-s) options are both given, any
character class can be used in set2. Otherwise, only the
upper are accepted in
set2, and then only if the corresponding character class
lower, respectively) is specified in the same
relative position in set1. Doing this specifies case conversion.
The class names are given below; an error results when an invalid class
name is given.
Letters and digits.
Printable characters, not including space.
Printable characters, including space.
Horizontal or vertical whitespace.
The syntax ‘[=c=]’ expands to all of the characters that are
equivalent to c, in no particular order. Equivalence classes are
a relatively recent invention intended to support non-English alphabets.
But there seems to be no standard way to define them or determine their
contents. Therefore, they are not fully implemented in GNU
each character’s equivalence class consists only of that character,
which is of no particular use.