Allow std::locale's to not have a messages facet installed (fixes some STLPort issues).

Allow match_results to handle singular iterators.
Update docs to match.

[SVN r57860]

doc/html/boost_regex/ref/bad_expression.html

@@ -27,7 +27,7 @@
 <a name="boost_regex.ref.bad_expression"></a><a class="link" href="bad_expression.html" title="bad_expression"> bad_expression</a>
 </h3></div></div></div>
 <a name="boost_regex.ref.bad_expression.synopsis"></a><h5>
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         <a class="link" href="bad_expression.html#boost_regex.ref.bad_expression.synopsis">Synopsis</a>
       </h5>
 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">pattern_except</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
@@ -54,7 +54,7 @@
 <span class="special">}</span> <span class="comment">// namespace boost
 </span></pre>
 <a name="boost_regex.ref.bad_expression.description"></a><h5>
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         <a class="link" href="bad_expression.html#boost_regex.ref.bad_expression.description">Description</a>
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 <pre class="programlisting"><span class="identifier">regex_error</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">s</span><span class="special">,</span> <span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">error_type</span> <span class="identifier">err</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">pos</span><span class="special">);</span>