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        <dd>
<span class="simpara">character with hex code hh</span></dd>

       </dt>

       <dt>

        <span class="term"><em class="emphasis">\ddd</em></span>

        <dd>
<span class="simpara">character with octal code ddd, or backreference</span></dd>

       </dt>

      </dl>

     </p>
    <p class="para">
     The precise effect of &quot;<i>\cx</i>&quot; is as follows: 
     if &quot;<i>x</i>&quot; is a lower case  letter, it is converted
     to upper case. Then bit 6 of the character (hex 40) is inverted. 
     Thus &quot;<i>\cz</i>&quot; becomes  hex 1A, but
     &quot;<i>\c{</i>&quot; becomes hex 3B, while &quot;<i>\c;</i>&quot;
     becomes hex 7B.
    </p>
    <p class="para">
     After &quot;<i>\x</i>&quot;, up to two hexadecimal digits are
     read (letters can be in upper or lower case).
     In <em class="emphasis">UTF-8 mode</em>, &quot;<i>\x{...}</i>&quot; is
     allowed, where the contents of the braces is a string of hexadecimal
     digits. It is interpreted as a UTF-8 character whose code number is the
     given hexadecimal number. The original hexadecimal escape sequence,
     <i>\xhh</i>, matches a two-byte UTF-8 character if the value
     is greater than 127.
    </p>
    <p class="para">
     After &quot;<i>\0</i>&quot; up to two further octal digits are read.
     In  both cases,  if  there are fewer than two digits, just those that
     are present are used. Thus the sequence &quot;<i>\0\x\07</i>&quot; 
     specifies two binary zeros followed by a BEL character. Make sure you
     supply two digits after the initial zero if the character
     that follows is itself an octal digit.
    </p>
    <p class="para">
     The handling of a backslash followed by a digit other than 0
     is complicated. Outside a character class, PCRE reads it
     and any following digits as a decimal number. If the  number
     is  less  than  10, or if there have been at least that many
     previous capturing left parentheses in the  expression,  the
     entire  sequence is taken as a <em class="emphasis">back</em> 
     <em class="emphasis">reference</em>. A description
     of how this works is given later, following  the  discussion
     of parenthesized subpatterns.
    </p>
    <p class="para">
     Inside a character  class,  or  if  the  decimal  number  is
     greater than 9 and there have not been that many capturing
     subpatterns, PCRE re-reads up to three octal digits following 
     the backslash, and generates a single byte from the
     least significant 8 bits of the value. Any subsequent digits
     stand for themselves.  For example:
    </p>
    <p class="para">
     <dl>

      <dt>

       <span class="term"><em class="emphasis">\040</em></span>

       <dd>
<span class="simpara">is another way of writing a space</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\40</em></span>

       <dd>

        <span class="simpara">
         is the same, provided there are fewer than 40
         previous capturing subpatterns
        </span>
       </dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\7</em></span>

       <dd>
<span class="simpara">is always a back reference</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\11</em></span>

       <dd>

        <span class="simpara">
         might be a back reference, or another way of
         writing a tab
        </span>
       </dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\011</em></span>

       <dd>
<span class="simpara">is always a tab</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\0113</em></span>

       <dd>
<span class="simpara">is a tab followed by the character &quot;3&quot;</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\113</em></span>

       <dd>

        <span class="simpara">
         is the character with octal code 113 (since there
         can be no more than 99 back references)
        </span>
       </dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\377</em></span>

       <dd>
<span class="simpara">is a byte consisting entirely of 1 bits</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\81</em></span>

       <dd>

        <span class="simpara">
         is either a back reference, or a binary zero
         followed by the two characters &quot;8&quot; and &quot;1&quot;
        </span>
       </dd>

      </dt>

     </dl>

    </p>
    <p class="para">
     Note that octal values of 100 or greater must not be
     introduced by a leading zero, because no more than three octal
     digits are ever read.
    </p>
    <p class="para">
     All the sequences that define a single byte value can  be
     used both inside and outside character classes. In addition,
     inside a character class, the sequence &quot;<i>\b</i>&quot;
     is interpreted as the backspace character (hex 08). Outside a character
     class it has a different meaning (see below).
    </p>
    <p class="para">
     The third use of backslash is for specifying generic
     character types:
    </p>
    <p class="para">
     <dl>

      <dt>

       <span class="term"><em class="emphasis">\d</em></span>

       <dd>
<span class="simpara">any decimal digit</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\D</em></span>

       <dd>
<span class="simpara">any character that is not a decimal digit</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\h</em></span>

       <dd>
<span class="simpara">any horizontal whitespace character (since PHP 5.2.4)</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\H</em></span>

       <dd>
<span class="simpara">any character that is not a horizontal whitespace character (since PHP 5.2.4)</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\s</em></span>

       <dd>
<span class="simpara">any whitespace character</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\S</em></span>

       <dd>
<span class="simpara">any character that is not a whitespace character</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\v</em></span>

       <dd>
<span class="simpara">any vertical whitespace character (since PHP 5.2.4)</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\V</em></span>

       <dd>
<span class="simpara">any character that is not a vertical whitespace character (since PHP 5.2.4)</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\w</em></span>

       <dd>
<span class="simpara">any &quot;word&quot; character</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\W</em></span>

       <dd>
<span class="simpara">any &quot;non-word&quot; character</span></dd>

      </dt>

     </dl>

    </p>
    <p class="para">
     Each pair of escape sequences partitions the complete set of
     characters into two disjoint sets. Any given character
     matches one, and only one, of each pair.
    </p>
    <p class="para">
     A &quot;word&quot; character is any letter or digit or the underscore
     character,  that  is,  any  character which can be part of a
     Perl &quot;<i>word</i>&quot;. The definition of letters and digits is  
     controlled by PCRE&#039;s character tables, and may vary if locale-specific
     matching is taking place. For example, in the &quot;fr&quot; (French) locale, some
     character codes greater than 128 are used for accented letters,
     and these are matched by <i>\w</i>.
    </p>
    <p class="para">
     These character type sequences can appear both inside and
     outside  character classes. They each match one character of
     the appropriate type. If the current matching  point is at
     the end of the subject string, all of them fail, since there
     is no character to match.
    </p>
    <p class="para">
     The fourth use of backslash is  for  certain  simple
     assertions. An assertion specifies a condition that has to be met
     at a particular point in  a match, without consuming any
     characters from the subject string. The use of subpatterns
     for more complicated assertions is described below. The
     backslashed assertions are
    </p>
    <p class="para">
     <dl>

      <dt>

       <span class="term"><em class="emphasis">\b</em></span>

       <dd>
<span class="simpara">word boundary</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\B</em></span>

       <dd>
<span class="simpara">not a word boundary</span></dd>

      </dt>

      <dt>

       <span class="term"><em class="emphasis">\A</em></span>

       <dd>
<span class="simpara">start of subject (independent of multiline mode)</span></dd>

      </dt>