File: groff.info,  Node: Strings,  Next: Conditionals and Loops,  Prev: Colors,  Up: GNU troff Reference

5.22 Strings
============

GNU 'troff' supports strings primarily for user convenience.
Conventionally, if one would define a macro only to interpolate a small
amount of text, without invoking requests or calling any other macros,
one defines a string instead.  Only one string is predefined by the
language.

 -- String: \*[.T]
     Contains the name of the output device (for example, 'utf8' or
     'pdf').

   The 'ds' request creates a string with a specified name and contents
and the '\*' escape sequence dereferences its name, interpolating its
contents.  If the string named by the '\*' escape sequence does not
exist, it is defined as empty, nothing is interpolated, and a warning in
category 'mac' is emitted.  *Note Warnings::, for information about the
enablement and suppression of warnings.

 -- Request: .ds name [contents]
 -- Request: .ds1 name [contents]
 -- Escape sequence: \*n
 -- Escape sequence: \*(nm
 -- Escape sequence: \*[name [arg1 arg2 ...]]
     Define a string called NAME with contents CONTENTS.  If NAME
     already exists as an alias, the target of the alias is redefined;
     see 'als' and 'rm' below.  If 'ds' is called with only one
     argument, NAME is defined as an empty string.  Otherwise, GNU
     'troff' stores CONTENTS in copy mode.(1)  (*note
     Strings-Footnote-1::)

     The '\*' escape sequence interpolates a previously defined string
     variable NAME (one-character name N, two-character name NM).  The
     bracketed interpolation form accepts arguments that are handled as
     macro arguments are; recall *note Calling Macros::.  In contrast to
     macro calls, however, if a closing bracket ']' occurs in a string
     argument, that argument must be enclosed in double quotes.  '\*' is
     interpreted even in copy mode.  When defining strings, argument
     interpolations must be escaped if they are to reference parameters
     from the calling context; *Note Parameters::.

          .ds cite (\\$1, \\$2)
          Gray codes are explored in \*[cite Morgan 1998].
              => Gray codes are explored in (Morgan, 1998).

     *Caution:* Unlike other requests, the second argument to the 'ds'
     request consumes the remainder of the input line, including
     trailing spaces.  This means that comments on a line with such a
     request can introduce unwanted space into a string when they are
     set off from the material they annotate, as is conventional.

          .ds H2O H\v'+.3m'\s'-2'2\v'-.3m'\s0O \" water

     Instead, place the comment on another line or put the comment
     escape sequence immediately adjacent to the last character of the
     string.

          .ds H2O H\v'+.3m'\s'-2'2\v'-.3m'\s0O\" water

     Ending string definitions (and appendments) with a comment, even an
     empty one, prevents unwanted space from creeping into them during
     source document maintenance.

          .ds author Alice Pleasance Liddell\"
          .ds empty \" might be appended to later with .as

     An initial neutral double quote '"' in CONTENTS is stripped to
     allow embedding of leading spaces.  Any other '"' is interpreted
     literally, but it is wise to use the special character escape
     sequence '\[dq]' instead if the string might be interpolated as
     part of a macro argument; see *note Calling Macros::.

          .ds salutation "         Yours in a white wine sauce,\"
          .ds c-var-defn "  char mydate[]=\[dq]2020-07-29\[dq];\"

     Strings are not limited to a single input line of text.  '\'
     works just as it does elsewhere.  The resulting string is stored
     _without_ the newlines.  Care is therefore required when
     interpolating strings while filling is disabled.

          .ds foo This string contains \
          text on multiple lines \
          of input.

     It is not possible to embed a newline in a string that will be
     interpreted as such when the string is interpolated.  To achieve
     that effect, use '\*' to interpolate a macro instead; see *note
     Punning Names::.

     Because strings are similar to macros, they too can be defined so
     as to suppress AT&T 'troff' compatibility mode when used; see *note
     Writing Macros:: and *note Compatibility Mode::.  The 'ds1' request
     defines a string such that compatibility mode is off when the
     string is later interpolated.  To be more precise, a "compatibility
     save" input token is inserted at the beginning of the string, and a
     "compatibility restore" input token at the end.

          .nr xxx 12345
          .ds aa The value of xxx is \\n[xxx].
          .ds1 bb The value of xxx is \\n[xxx].
          .
          .cp 1
          .
          \*(aa
              error-> warning: register '[' not defined
              => The value of xxx is 0xxx].
          \*(bb
              => The value of xxx is 12345.

 -- Request: .as name [contents]
 -- Request: .as1 name [contents]
     The 'as' request is similar to 'ds' but appends CONTENTS to the
     string stored as NAME instead of redefining it.  If NAME doesn't
     exist yet, it is created.  If 'as' is called with only one
     argument, no operation is performed (beyond dereferencing the
     string).

          .as salutation " with shallots, onions and garlic,\"

     The 'as1' request is similar to 'as', but compatibility mode is
     switched off when the appended portion of the string is later
     interpolated.  To be more precise, a "compatibility save" input
     token is inserted at the beginning of the appended string, and a
     "compatibility restore" input token at the end.

   Several requests exist to perform rudimentary string operations.
Strings can be queried ('length') and modified ('chop', 'substring',
'stringup', 'stringdown'), and their names can be manipulated through
renaming, removal, and aliasing ('rn', 'rm', 'als').

 -- Request: .length reg anything
     Compute the number of characters of ANYTHING and store the count in
     the register REG.  If REG doesn't exist, it is created.  ANYTHING
     is read in copy mode.

          .ds xxx abcd\h'3i'efgh
          .length yyy \*[xxx]
          \n[yyy]
              => 14

 -- Request: .chop object
     Remove the last character from the macro, string, or diversion
     named OBJECT.  This is useful for removing the newline from the end
     of a diversion that is to be interpolated as a string.  This
     request can be used repeatedly on the same OBJECT; see *note Gtroff
     Internals::, for details on nodes inserted additionally by GNU
     'troff'.

 -- Request: .substring str start [end]
     Replace the string named STR with its substring bounded by the
     indices START and END, inclusively.  The first character in the
     string has index 0.  If END is omitted, it is implicitly set to the
     largest valid value (the string length minus one).  Negative
     indices count backward from the end of the string: the last
     character has index -1, the character before the last has index -2,
     and so on.

          .ds xxx abcdefgh
          .substring xxx 1 -4
          \*[xxx]
              => bcde
          .substring xxx 2
          \*[xxx]
              => de

 -- Request: .stringdown str
 -- Request: .stringup str
     Alter the string named STR by replacing each of its bytes with its
     lowercase ('stringdown') or uppercase ('stringup') version (if one
     exists).  Special characters in the string will often transform in
     the expected way due to the regular naming convention for accented
     characters.  When they do not, use substrings and/or catenation.

          .ds resume R\['e]sum\['e]
          \*[resume]
          .stringdown resume
          \*[resume]
          .stringup resume
          \*[resume]
              => Résumé résumé RÉSUMÉ

   (In practice, we would end the 'ds' request with a comment escape
'\"' to prevent space from creeping into the definition during source
document maintenance.)

 -- Request: .rn old new
     Rename the request, macro, diversion, or string OLD to NEW.

 -- Request: .rm name
     Remove the request, macro, diversion, or string NAME.  GNU 'troff'
     treats subsequent invocations as if the name had never been
     defined.

 -- Request: .als new old
     Create an alias NEW for the existing request, string, macro, or
     diversion object named OLD, causing the names to refer to the same
     stored object.  If OLD is undefined, a warning in category 'mac' is
     produced, and the request is ignored.  *Note Warnings::, for
     information about the enablement and suppression of warnings.

     To understand how the 'als' request works, consider two different
     storage pools: one for objects (macros, strings, etc.), and another
     for names.  As soon as an object is defined, GNU 'troff' adds it to
     the object pool, adds its name to the name pool, and creates a link
     between them.  When 'als' creates an alias, it adds a new name to
     the name pool that gets linked to the same object as the old name.

     Now consider this example.

          .de foo
          ..
          .
          .als bar foo
          .
          .de bar
          .  foo
          ..
          .
          .bar
              error-> input stack limit exceeded (probable infinite
              error-> loop)

     In the above, 'bar' remains an _alias_--another name for--the
     object referred to by 'foo', which the second 'de' request
     replaces.  Alternatively, imagine that the 'de' request
     _dereferences_ its argument before replacing it.  Either way, the
     result of calling 'bar' is a recursive loop that finally leads to
     an error.  *Note Writing Macros::.

     To remove an alias, call 'rm' on its name.  The object itself is
     not destroyed until it has no more names.

     When a request, macro, string, or diversion is aliased,
     redefinitions and appendments "write through" alias names.  To
     replace an alias with a separately defined object, you must use the
     'rm' request on its name first.