File: coreutils.info,  Node: Character arrays,  Next: Translating,  Up: tr invocation

9.1.1 Specifying arrays of characters
-------------------------------------

The STRING1 and STRING2 operands are not regular expressions, even
though they may look similar.  Instead, they merely represent arrays of
characters.  As a GNU extension to POSIX, an empty string operand
represents an empty array of characters.

   The interpretation of STRING1 and STRING2 depends on locale.  GNU
‘tr’ fully supports only safe single-byte locales, where each possible
input byte represents a single character.  Unfortunately, this means GNU
‘tr’ will not handle commands like ‘tr ö Ł’ the way you might expect,
since (assuming a UTF-8 encoding) this is equivalent to ‘tr '\303\266'
'\305\201'’ and GNU ‘tr’ will simply transliterate all ‘\303’ bytes to
‘\305’ bytes, etc.  POSIX does not clearly specify the behavior of ‘tr’
in locales where characters are represented by byte sequences instead of
by individual bytes, or where data might contain invalid bytes that are
encoding errors.  To avoid problems in this area, you can run ‘tr’ in a
safe single-byte locale by using a shell command like ‘LC_ALL=C tr’
instead of plain ‘tr’.

   Although most characters simply represent themselves in STRING1 and
STRING2, the strings can contain shorthands listed below, for
convenience.  Some shorthands can be used only in STRING1 or STRING2, as
noted below.

Backslash escapes

     The following backslash escape sequences are recognized:

     ‘\a’
          Bell (BEL, Control-G).
     ‘\b’
          Backspace (BS, Control-H).
     ‘\f’
          Form feed (FF, Control-L).
     ‘\n’
          Newline (LF, Control-J).
     ‘\r’
          Carriage return (CR, Control-M).
     ‘\t’
          Tab (HT, Control-I).
     ‘\v’
          Vertical tab (VT, Control-K).
     ‘\OOO’
          The eight-bit byte with the value given by OOO, which is the
          longest sequence of one to three octal digits following the
          backslash.  For portability, OOO should represent a value that
          fits in eight bits.  As a GNU extension to POSIX, if the value
          would not fit, then only the first two digits of OOO are used,
          e.g., ‘\400’ is equivalent to ‘\0400’ and represents a
          two-byte sequence.
     ‘\\’
          A backslash.

     It is an error if no character follows an unescaped backslash.  As
     a GNU extension, a backslash followed by a character not listed
     above is interpreted as that character, removing any special
     significance; this can be used to escape the characters ‘[’ and ‘-’
     when they would otherwise be special.

Ranges

     The notation ‘M-N’ expands to the characters from M through N, in
     ascending order.  M should not collate after N; if it does, an
     error results.  As an example, ‘0-9’ is the same as ‘0123456789’.

     GNU ‘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
     fully 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.  One 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.

Repeated characters

     The notation ‘[C*N]’ in STRING2 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 ARRAY2
     as long as ARRAY1.  If N begins with ‘0’, it is interpreted in
     octal, otherwise in decimal.  A zero-valued N is treated as if it
     were absent.

Character classes

     The notation ‘[:CLASS:]’ expands to all characters in the
     (predefined) class CLASS.  When the ‘--delete’ (‘-d’) and
     ‘--squeeze-repeats’ (‘-s’) options are both given, any character
     class can be used in STRING2.  Otherwise, only the character
     classes ‘lower’ and ‘upper’ are accepted in STRING2, and then only
     if the corresponding character class (‘upper’ and ‘lower’,
     respectively) is specified in the same relative position in
     STRING1.  Doing this specifies case conversion.  Except for case
     conversion, a class’s characters appear in no particular order.
     The class names are given below; an error results when an invalid
     class name is given.

     ‘alnum’
          Letters and digits.
     ‘alpha’
          Letters.
     ‘blank’
          Horizontal whitespace.
     ‘cntrl’
          Control characters.
     ‘digit’
          Digits.
     ‘graph’
          Printable characters, not including space.
     ‘lower’
          Lowercase letters.
     ‘print’
          Printable characters, including space.
     ‘punct’
          Punctuation characters.
     ‘space’
          Horizontal or vertical whitespace.
     ‘upper’
          Uppercase letters.
     ‘xdigit’
          Hexadecimal digits.

Equivalence classes

     The syntax ‘[=C=]’ expands to all characters equivalent to C, in no
     particular order.  These equivalence classes are allowed in STRING2
     only when ‘--delete’ (‘-d’) and ‘--squeeze-repeats’ ‘-s’ are both
     given.

     Although equivalence classes are intended to support non-English
     alphabets, there seems to be no standard way to define them or
     determine their contents.  Therefore, they are not fully
     implemented in GNU ‘tr’; each character’s equivalence class
     consists only of that character, which is of no particular use.