The operands string1 and string2 (if specified) define two arrays
of characters. The constructs in the following list can be used
to specify characters or single-character collating elements. If
any of the constructs result in multi-character collating
elements, tr shall exclude, without a diagnostic, those multi-
character elements from the resulting array.
character Any character not described by one of the conventions
below shall represent itself.
\octal Octal sequences can be used to represent characters
with specific coded values. An octal sequence shall
consist of a <backslash> followed by the longest
sequence of one, two, or three-octal-digit characters
(01234567). The sequence shall cause the value whose
encoding is represented by the one, two, or three-digit
octal integer to be placed into the array. Multi-byte
characters require multiple, concatenated escape
sequences of this type, including the leading
<backslash> for each byte.
\character
The <backslash>-escape sequences in the Base
Definitions volume of POSIX.1‐2017, Table 5-1, Escape
Sequences and Associated Actions ('\\', '\a', '\b',
'\f', '\n', '\r', '\t', '\v') shall be supported. The
results of using any other character, other than an
octal digit, following the <backslash> are unspecified.
Also, if there is no character following the
<backslash>, the results are unspecified.
c-c In the POSIX locale, this construct shall represent the
range of collating elements between the range endpoints
(as long as neither endpoint is an octal sequence of
the form \octal), inclusive, as defined by the
collation sequence. The characters or collating
elements in the range shall be placed in the array in
ascending collation sequence. If the second endpoint
precedes the starting endpoint in the collation
sequence, it is unspecified whether the range of
collating elements is empty, or this construct is
treated as invalid. In locales other than the POSIX
locale, this construct has unspecified behavior.
If either or both of the range endpoints are octal
sequences of the form \octal, this shall represent the
range of specific coded values between the two range
endpoints, inclusive.
[:class:] Represents all characters belonging to the defined
character class, as defined by the current setting of
the LC_CTYPE locale category. The following character
class names shall be accepted when specified in
string1:
alnum blank digit lower punct upper
alpha cntrl graph print space xdigit
In addition, character class expressions of the form
[:name:] shall be recognized in those locales where the
name keyword has been given a charclass definition in
the LC_CTYPE category.
When both the -d and -s options are specified, any of
the character class names shall be accepted in string2.
Otherwise, only character class names lower or upper
are valid in string2 and then only if the corresponding
character class (upper and lower, respectively) is
specified in the same relative position in string1.
Such a specification shall be interpreted as a request
for case conversion. When [:lower:] appears in string1
and [:upper:] appears in string2, the arrays shall
contain the characters from the toupper mapping in the
LC_CTYPE category of the current locale. When [:upper:]
appears in string1 and [:lower:] appears in string2,
the arrays shall contain the characters from the
tolower mapping in the LC_CTYPE category of the current
locale. The first character from each mapping pair
shall be in the array for string1 and the second
character from each mapping pair shall be in the array
for string2 in the same relative position.
Except for case conversion, the characters specified by
a character class expression shall be placed in the
array in an unspecified order.
If the name specified for class does not define a valid
character class in the current locale, the behavior is
undefined.
[=equiv=] Represents all characters or collating elements
belonging to the same equivalence class as equiv, as
defined by the current setting of the LC_COLLATE locale
category. An equivalence class expression shall be
allowed only in string1, or in string2 when it is being
used by the combined -d and -s options. The characters
belonging to the equivalence class shall be placed in
the array in an unspecified order.
[x*n] Represents n repeated occurrences of the character x.
Because this expression is used to map multiple
characters to one, it is only valid when it occurs in
string2. If n is omitted or is zero, it shall be
interpreted as large enough to extend the string2-based
sequence to the length of the string1-based sequence.
If n has a leading zero, it shall be interpreted as an
octal value. Otherwise, it shall be interpreted as a
decimal value.
When the -d option is not specified:
* If string2 is present, each input character found in the
array specified by string1 shall be replaced by the character
in the same relative position in the array specified by
string2. If the array specified by string2 is shorter that
the one specified by string1, or if a character occurs more
than once in string1, the results are unspecified.
* If the -C option is specified, the complements of the
characters specified by string1 (the set of all characters in
the current character set, as defined by the current setting
of LC_CTYPE, except for those actually specified in the
string1 operand) shall be placed in the array in ascending
collation sequence, as defined by the current setting of
LC_COLLATE.
* If the -c option is specified, the complement of the values
specified by string1 shall be placed in the array in
ascending order by binary value.
* Because the order in which characters specified by character
class expressions or equivalence class expressions is
undefined, such expressions should only be used if the intent
is to map several characters into one. An exception is case
conversion, as described previously.
When the -d option is specified:
* Input characters found in the array specified by string1
shall be deleted.
* When the -C option is specified with -d, all characters
except those specified by string1 shall be deleted. The
contents of string2 are ignored, unless the -s option is also
specified.
* When the -c option is specified with -d, all values except
those specified by string1 shall be deleted. The contents of
string2 shall be ignored, unless the -s option is also
specified.
* The same string cannot be used for both the -d and the -s
option; when both options are specified, both string1 (used
for deletion) and string2 (used for squeezing) shall be
required.
When the -s option is specified, after any deletions or
translations have taken place, repeated sequences of the same
character shall be replaced by one occurrence of the same
character, if the character is found in the array specified by
the last operand. If the last operand contains a character class,
such as the following example:
tr -s '[:space:]'
the last operand's array shall contain all of the characters in
that character class. However, in a case conversion, as described
previously, such as:
tr -s '[:upper:]' '[:lower:]'
the last operand's array shall contain only those characters
defined as the second characters in each of the toupper or
tolower character pairs, as appropriate.
An empty string used for string1 or string2 produces undefined
results.
