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<hr>
<h3 class="section" id="Identifiers-1">5.5 Identifiers</h3>
<a class="index-entry-id" id="index-identifiers"></a>
<p>An <em class="dfn">identifier</em> labels a GNU <code class="code">troff</code> datum such as a register,
name (macro, string, or diversion), typeface, color, special character,
character class, environment, or stream. Valid identifiers consist of
one or more ordinary characters.
<a class="index-entry-id" id="index-ordinary-character"></a>
<a class="index-entry-id" id="index-character_002c-ordinary"></a>
An <i class="slanted">ordinary character</i> is an input character that is not the
escape character, a leader, tab, newline, or invalid as GNU <code class="code">troff</code>
input.
</p>
<a class="index-entry-id" id="index-invalid-input-characters"></a>
<a class="index-entry-id" id="index-input-characters_002c-invalid"></a>
<a class="index-entry-id" id="index-characters_002c-invalid-input"></a>
<a class="index-entry-id" id="index-Unicode"></a>
<p>Invalid input characters are a subset of control characters (from the
sets “C0 Controls” and “C1 Controls” as Unicode describes them).
When GNU <code class="code">troff</code> encounters one in an identifier, it produces a
warning in category ‘<samp class="samp">input</samp>’ (see <a class="pxref" href="Warnings.html">Warnings</a>). They are removed
during interpretation: an identifier ‘<samp class="samp">foo</samp>’, followed by an invalid
character and then ‘<samp class="samp">bar</samp>’, is processed as ‘<samp class="samp">foobar</samp>’.
</p>
<p>On a machine using the ISO 646, 8859, or 10646 character encodings,
invalid input characters are <code class="code">0x00</code>, <code class="code">0x08</code>, <code class="code">0x0B</code>,
<code class="code">0x0D</code>–<code class="code">0x1F</code>, and <code class="code">0x80</code>–<code class="code">0x9F</code>. On an
<abbr class="acronym">EBCDIC</abbr> host, they are <code class="code">0x00</code>–<code class="code">0x01</code>, <code class="code">0x08</code>,
<code class="code">0x09</code>, <code class="code">0x0B</code>, <code class="code">0x0D</code>–<code class="code">0x14</code>,
<code class="code">0x17</code>–<code class="code">0x1F</code>, and
<code class="code">0x30</code>–<code class="code">0x3F</code>.<a class="footnote" id="DOCF40" href="groff.html_fot.html#FOOT40"><sup>40</sup></a> Some of these code points are used
by GNU <code class="code">troff</code> internally, making it non-trivial to extend the
program to accept UTF-8 or other encodings that use characters from
these ranges.<a class="footnote" id="DOCF41" href="groff.html_fot.html#FOOT41"><sup>41</sup></a>
</p>
<p>Thus, the identifiers ‘<samp class="samp">br</samp>’, ‘<samp class="samp">PP</samp>’, ‘<samp class="samp">end-list</samp>’,
‘<samp class="samp">ref*normal-print</samp>’, ‘<samp class="samp">|</samp>’, ‘<samp class="samp">@_</samp>’, and ‘<samp class="samp">!"#$%'()*+,-./</samp>’
are all valid. Discretion should be exercised to prevent confusion.
Identifiers starting with ‘<samp class="samp">(</samp>’ or ‘<samp class="samp">[</samp>’ require care.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.nr x 9
.nr y 1
.nr (x 2
.nr [y 3
.nr sum1 (\n(x + \n[y])
error→ a space character is not allowed in an escape
error→ sequence parameter
A:2+3=\n[sum1]
.nr sum2 (\n((x + \n[[y])
B:2+3=\n[sum2]
.nr sum3 (\n[(x] + \n([y)
C:2+3=\n[sum3]
⇒ A:2+3=1 B:2+3=5 C:2+3=5
</pre></div></div>
<a class="index-entry-id" id="index-_005d_002c-as-part-of-an-identifier"></a>
<p>An identifier with a closing bracket (‘<samp class="samp">]</samp>’) in its name can’t be
accessed with bracket-form escape sequences that expect an identifier as
a parameter. For example, ‘<samp class="samp">\[foo]]</samp>’ accesses the glyph ‘<samp class="samp">foo</samp>’,
followed by ‘<samp class="samp">]</samp>’ in whatever the surrounding context is, whereas
‘<samp class="samp">\C'foo]'</samp>’ formats a glyph named ‘<samp class="samp">foo]</samp>’. Similarly, the
identifier ‘<samp class="samp">(</samp>’ can’t be interpolated <em class="emph">except</em> with bracket
forms.
</p>
<a class="index-entry-id" id="index-refer_002c-and-macro-names-starting-with-_005b-or-_005d"></a>
<a class="index-entry-id" id="index-_005b_002c-macro-names-starting-with_002c-and-refer"></a>
<a class="index-entry-id" id="index-_005d_002c-macro-names-starting-with_002c-and-refer"></a>
<a class="index-entry-id" id="index-macro-names_002c-starting-with-_005b-or-_005d_002c-and-refer"></a>
<p>If you begin a macro, string, or diversion name with either of the
characters ‘<samp class="samp">[</samp>’ or ‘<samp class="samp">]</samp>’, you foreclose use of the <code class="code">grefer</code>
preprocessor, which recognizes ‘<samp class="samp">.[</samp>’ and ‘<samp class="samp">.]</samp>’ as bibliographic
reference delimiters.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_005cA_0027anything_0027"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\A<code class="code">'</code></code><span class="r"><i class="slanted">anything</i></span><code class="t"><code class="code">'</code></code></strong><a class="copiable-link" href='#index-_005cA_0027anything_0027'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_005cA"></a>
<p>Interpolate 1 if <var class="var">anything</var> is a valid identifier, and 0
otherwise. The delimiter need not be a neutral apostrophe; see
<a class="ref" href="Delimiters.html">Delimiters</a>. Because invalid input characters are removed (see
above), invalid identifiers are empty or contain spaces, tabs, or
newlines.
</p>
<p>You can employ <code class="code">\A</code> to validate a macro argument before using it to
construct another escape sequence or identifier.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.\" usage: .init-coordinate-pair name val1 val2
.\" Create a coordinate pair where name!x=val1 and
.\" name!y=val2.
.de init-coordinate-pair
. if \A'\\$1' \{\
. if \B'\\$2' .nr \\$1!x \\$2
. if \B'\\$3' .nr \\$1!y \\$3
. \}
..
.init-coordinate-pair center 5 10
The center is at (\n[center!x], \n[center!y]).
.init-coordinate-pair "poi→nt" trash garbage \" ignored
.init-coordinate-pair point trash garbage \" ignored
⇒ The center is at (5, 10).
</pre></div></div>
<p>In this example, we also validated the numeric arguments; the registers
‘<samp class="samp">point!x</samp>’ and ‘<samp class="samp">point!y</samp>’ remain undefined. See <a class="xref" href="Numeric-Expressions.html">Numeric Expressions</a> for the <code class="code">\B</code> escape sequence.
</p></dd></dl>
<a class="index-entry-id" id="index-undefined-identifiers"></a>
<a class="index-entry-id" id="index-identifiers_002c-undefined"></a>
<p>How GNU <code class="code">troff</code> handles the interpretation of an undefined
identifier depends on the context. There is no way to invoke an
undefined request; such syntax is interpreted as a macro call instead.
If the identifier is interpreted as a string, macro, or diversion, GNU
<code class="code">troff</code> emits a warning in category ‘<samp class="samp">mac</samp>’, defines it as
empty, and interpolates nothing. If the identifier is interpreted as a
register, GNU <code class="code">troff</code> emits a warning in category ‘<samp class="samp">reg</samp>’,
initializes it to zero, and interpolates that value. See <a class="xref" href="Warnings.html">Warnings</a>,
<a class="ref" href="Interpolating-Registers.html">Interpolating Registers</a>, and <a class="ref" href="Strings.html">Strings</a>. Attempting to use an
undefined typeface, special character, color, character class,
environment, or stream generally provokes an error diagnostic.
</p>
<a class="index-entry-id" id="index-name-space_002c-common_002c-of-macros_002c-diversions_002c-and-strings"></a>
<a class="index-entry-id" id="index-common-name-space-of-macros_002c-diversions_002c-and-strings"></a>
<a class="index-entry-id" id="index-macros_002c-shared-name-space-with-strings-and-diversions"></a>
<a class="index-entry-id" id="index-strings_002c-shared-name-space-with-macros-and-diversions"></a>
<a class="index-entry-id" id="index-diversions_002c-shared-name-space-with-macros-and-strings"></a>
<p>Identifiers for requests, macros, strings, and diversions share one name
space; special characters and character classes another. No other
object types do.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de xxx
. nop foo
..
.di xxx
bar
.br
.di
.
.xxx
⇒ bar
</pre></div></div>
<p>The foregoing example shows that GNU <code class="code">troff</code> reuses the identifier
‘<samp class="samp">xxx</samp>’, changing it from a macro to a diversion. No warning is
emitted, and the previous contents of ‘<samp class="samp">xxx</samp>’ are lost.
</p>
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