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<h4 class="subsection">10.3.6 POSIX Regexp Matching Cleanup</h4>

<p>When you are finished using a compiled regular expression, you can
free the storage it uses by calling <code>regfree</code>.

<!-- regex.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Function: void <b>regfree</b> (<var>regex_t *compiled</var>)<var><a name="index-regfree-882"></a></var><br>
<blockquote><p>Calling <code>regfree</code> frees all the storage that <code>*</code><var>compiled</var>
points to.  This includes various internal fields of the <code>regex_t</code>
structure that aren't documented in this manual.

        <p><code>regfree</code> does not free the object <code>*</code><var>compiled</var> itself. 
</p></blockquote></div>

   <p>You should always free the space in a <code>regex_t</code> structure with
<code>regfree</code> before using the structure to compile another regular
expression.

   <p>When <code>regcomp</code> or <code>regexec</code> reports an error, you can use
the function <code>regerror</code> to turn it into an error message string.

<!-- regex.h -->
<!-- POSIX.2 -->
<div class="defun">
&mdash; Function: size_t <b>regerror</b> (<var>int errcode, const regex_t *restrict compiled, char *restrict buffer, size_t length</var>)<var><a name="index-regerror-883"></a></var><br>
<blockquote><p>This function produces an error message string for the error code
<var>errcode</var>, and stores the string in <var>length</var> bytes of memory
starting at <var>buffer</var>.  For the <var>compiled</var> argument, supply the
same compiled regular expression structure that <code>regcomp</code> or
<code>regexec</code> was working with when it got the error.  Alternatively,
you can supply <code>NULL</code> for <var>compiled</var>; you will still get a
meaningful error message, but it might not be as detailed.

        <p>If the error message can't fit in <var>length</var> bytes (including a
terminating null character), then <code>regerror</code> truncates it. 
The string that <code>regerror</code> stores is always null-terminated
even if it has been truncated.

        <p>The return value of <code>regerror</code> is the minimum length needed to
store the entire error message.  If this is less than <var>length</var>, then
the error message was not truncated, and you can use it.  Otherwise, you
should call <code>regerror</code> again with a larger buffer.

        <p>Here is a function which uses <code>regerror</code>, but always dynamically
allocates a buffer for the error message:

     <pre class="smallexample">          char *get_regerror (int errcode, regex_t *compiled)
          {
            size_t length = regerror (errcode, compiled, NULL, 0);
            char *buffer = xmalloc (length);
            (void) regerror (errcode, compiled, buffer, length);
            return buffer;
          }
</pre>
        </blockquote></div>

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