Escape back slashes

Fixes Trac #12096.

https://svn.boost.org/trac/boost/ticket/12096

doc/html/boost_regex/syntax/perl_syntax.html

@@ -578,12 +578,12 @@
 <tr>
 <td>
                 <p>
-                  <code class="literal"><br> </code>
+                  <code class="literal">\n</code>
                 </p>
               </td>
 <td>
                 <p>
-                  <code class="literal"><br> </code>
+                  <code class="literal">\n</code>
                 </p>
               </td>
 </tr>

doc/syntax_perl.qbk

@@ -299,24 +299,24 @@ The following escape sequences are all synonyms for single characters:
 
 [table
 [[Escape][Character]]
-[[[^\a]][[^\a]]]
-[[[^\e]][[^0x1B]]]
-[[[^\f]][[^\f]]]
-[[[^\n]][[^\n]]]
-[[[^\r]][[^\r]]]
-[[[^\t]][[^\t]]]
-[[[^\v]][[^\v]]]
-[[[^\b]][[^\b] (but only inside a character class declaration).]]
-[[[^\cX]][An ASCII escape sequence - the character whose code point is X % 32]]
-[[[^\xdd]][A hexadecimal escape sequence - matches the single character whose 
+[[[^\\a]][[^\\a]]]
+[[[^\\e]][[^0x1B]]]
+[[[^\\f]][[^\\f]]]
+[[[^\\n]][[^\\n]]]
+[[[^\\r]][[^\\r]]]
+[[[^\\t]][[^\\t]]]
+[[[^\\v]][[^\\v]]]
+[[[^\\b]][[^\\b] (but only inside a character class declaration).]]
+[[[^\\cX]][An ASCII escape sequence - the character whose code point is X % 32]]
+[[[^\\xdd]][A hexadecimal escape sequence - matches the single character whose 
       code point is 0xdd.]]
-[[[^\x{dddd}]][A hexadecimal escape sequence - matches the single character whose 
+[[[^\\x{dddd}]][A hexadecimal escape sequence - matches the single character whose 
       code point is 0xdddd.]]
-[[[^\0ddd]][An octal escape sequence - matches the single character whose 
+[[[^\\0ddd]][An octal escape sequence - matches the single character whose 
    code point is 0ddd.]]
-[[[^\N{name}]][Matches the single character which has the 
+[[[^\\N{name}]][Matches the single character which has the 
       [link boost_regex.syntax.collating_names symbolic name] /name/.  
-      For example [^\N{newline}] matches the single character \\n.]]
+      For example [^\\N{newline}] matches the single character \\n.]]
 ]      
  
 [h5 "Single character" character classes:]