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<div class="subsection-level-extent" id="Using-Escape-Sequences">
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<h4 class="subsection" id="Using-Escape-Sequences-1">5.6.4 Using Escape Sequences</h4>
<a class="index-entry-id" id="index-using-escape-sequences"></a>
<a class="index-entry-id" id="index-escape-sequences"></a>
<p>Whereas requests must occur on control lines, escape sequences can occur
intermixed with text and may appear in arguments to requests, macros,
and other escape sequences.
<a class="index-entry-id" id="index-_005c"></a>
An escape sequence is introduced by the escape character, a backslash
<code class="code">\</code> (but see the <code class="code">ec</code> request below). The next character
selects the escape’s function.
</p>
<p>Escape sequences vary in length. Some take an argument, and of those,
some have different syntactical forms for a one-character,
two-character, or arbitrary-length argument. Others accept <em class="emph">only</em>
an arbitrary-length argument. In the former scheme, a one-character
argument follows the function character immediately, an opening
parenthesis ‘<samp class="samp">(</samp>’ introduces a two-character argument (no closing
parenthesis is used), and an argument of arbitrary length is enclosed in
brackets ‘<samp class="samp">[]</samp>’. In the latter scheme, the user selects a delimiter
character. A few escape sequences are idiosyncratic, and support both
of the foregoing conventions (<code class="code">\s</code>), designate their own
termination sequence (<code class="code">\?</code>), consume input until the next newline
(<code class="code">\!</code>, <code class="code">\"</code>, <code class="code">\#</code>), or support an additional modifier
character (<code class="code">\s</code> again, and <code class="code">\n</code>). As with requests, use of
some escape sequences in source documents may interact poorly with a
macro package you use; consult its documentation to learn of “safe”
sequences or alternative facilities it provides to achieve the desired
result.
</p>
<p>If an escape character is followed by a character that does not
identify a defined operation, the escape character is ignored (producing
a diagnostic of the ‘<samp class="samp">escape</samp>’ warning category, which is not enabled
by default) and the following character is processed normally.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">$ groff -Tps -ww
.nr N 12
.ds co white
.ds animal elephant
I have \fI\nN \*(co \*[animal]s,\f[]
said \P.\&\~Pseudo Pachyderm.
error→ warning: escape character ignored before 'P'
⇒ I have <i class="slanted">12 white elephants,</i> said P. Pseudo Pachyderm.
</pre></div></div>
<p>Escape sequence interpolation is of higher precedence than escape
sequence argument interpretation. This rule affords flexibility in
using escape sequences to construct parameters to other escape
sequences.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ds family C\" Courier
.ds style I\" oblique
Choice a typeface \f(\*[family]\*[style]wisely.
⇒ Choose a typeface <i class="slanted">wisely.</i>
</pre></div></div>
<p>In the above, the syntax form ‘<samp class="samp">\f(</samp>’ accepts only two characters for
an argument; the example works because the subsequent escape sequences
are interpolated before the selection escape sequence argument is
processed, and strings <code class="code">family</code> and <code class="code">style</code> interpolate one
character each.<a class="footnote" id="DOCF46" href="groff.html_fot.html#FOOT46"><sup>46</sup></a>
</p>
<p>The escape character is nearly always interpreted when encountered; it
is therefore desirable to have a way to interpolate it, disable it, or
change it.
</p>
<a class="index-entry-id" id="index-formatting-the-escape-character-_0028_005ce_0029"></a>
<a class="index-entry-id" id="index-escape-character_002c-formatting-_0028_005ce_0029"></a>
<dl class="first-deffn">
<dt class="deffn" id="index-_005ce-1"><span class="category-def">Escape sequence: </span><span><strong class="def-name"><code class="t">\e</code><span class="r"><i class="slanted"></i></span><code class="t"></code></strong><a class="copiable-link" href='#index-_005ce-1'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_005ce"></a>
<p>Interpolate the escape character.
</p></dd></dl>
<a class="index-entry-id" id="index-formatting-a-backslash-glyph-_0028_005c_005brs_005d_0029"></a>
<a class="index-entry-id" id="index-backslash-glyph_002c-formatting-_0028_005c_005brs_005d_0029"></a>
<p>The <code class="code">\[rs]</code> special character escape sequence formats a backslash
glyph. In macro and string definitions, the input sequences <code class="code">\\</code>
and <code class="code">\E</code> defer interpretation of escape sequences. See <a class="xref" href="Copy-Mode.html">Copy Mode</a>.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002eeo"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.eo</code></strong><a class="copiable-link" href='#index-_002eeo'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-eo"></a>
<a class="index-entry-id" id="index-disabling-_005c-_0028eo_0029"></a>
<a class="index-entry-id" id="index-_005c_002c-disabling-_0028eo_0029"></a>
<p>Disable the escape mechanism except in copy mode. Once this request is
invoked, no input character is recognized as starting an escape
sequence in interpretation mode.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002eec"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.ec</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">o</i></span>]</var><a class="copiable-link" href='#index-_002eec'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-ec"></a>
<a class="index-entry-id" id="index-escape-character_002c-changing-_0028ec_0029"></a>
<a class="index-entry-id" id="index-character_002c-escape_002c-changing-_0028ec_0029"></a>
<p>Recognize the ordinary character <var class="var">o</var> as the escape character.
If <var class="var">o</var> is absent or invalid, the default escape character
‘<samp class="samp">\</samp>’ is selected.
</p></dd></dl>
<p>Switching escape sequence interpretation off to define a macro and back
on afterward can obviate the need to double the escape character within
the definition. See <a class="xref" href="Writing-Macros.html">Writing Macros</a>. This technique is not available
if your macro needs to interpolate values at the time it is
<em class="emph">defined</em>—but many do not.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.\" simplified `BR` macro from the man(7) macro package
.eo
.de BR
. ds result \&
. while (\n[.$] >= 2) \{\
. as result \fB\$1\fR\$2\"
. shift 2
. \}
. if \n[.$] .as result \fB\$1\"
\*[result]
. rm result
. ft R
..
.ec
</pre></div></div>
<dl class="first-deffn">
<dt class="deffn" id="index-_002eecs"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.ecs</code></strong><a class="copiable-link" href='#index-_002eecs'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-ecs"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_002eecr"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.ecr</code></strong><a class="copiable-link" href='#index-_002eecr'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-ecr"></a>
<p>The <code class="code">ecs</code> request stores the escape character for recall with
<code class="code">ecr</code>. <code class="code">ecr</code> sets the escape character to ‘<samp class="samp">\</samp>’ if none
has been saved.
</p>
<p>Use these requests together to temporarily change the escape character.
</p></dd></dl>
<p>Using a different escape character, or disabling it, when calling macros
not under your control will likely cause errors, since GNU <code class="code">troff</code>
has no mechanism to “intern” macros—that is, to convert a macro
definition into a form independent of its
representation.<a class="footnote" id="DOCF47" href="groff.html_fot.html#FOOT47"><sup>47</sup></a> When a
macro is called, its contents are interpreted literally.
</p>
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