Source of: /manual/en/regexp.reference.backslash.php
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<div id="regexp.reference.backslash" class="section">
<h2 class="title">Backslash</h2>
<p class="para">
The backslash character has several uses. Firstly, if it is
followed by a non-alphanumeric character, it takes away any
special meaning that character may have. This use of
backslash as an escape character applies both inside and
outside character classes.
</p>
<p class="para">
For example, if you want to match a "*" character, you write
"\*" in the pattern. This applies whether or not the
following character would otherwise be interpreted as a
meta-character, so it is always safe to precede a non-alphanumeric
with "\" to specify that it stands for itself. In
particular, if you want to match a backslash, you write "\\".
</p>
<blockquote><p><b class="note">Note</b>:
Single and double quoted PHP <a href="language.types.string.php#language.types.string.syntax" class="link">strings</a> have special
meaning of backslash. Thus if \ has to be matched with a regular
expression \\, then "\\\\" or '\\\\' must be used in PHP code.
<br />
</p></blockquote>
<p class="para">
If a pattern is compiled with the
<a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_EXTENDED</a> option,
whitespace in the pattern (other than in a character class) and
characters between a "#" outside a character class and the next newline
character are ignored. An escaping backslash can be used to include a
whitespace or "#" character as part of the pattern.
</p>
<p class="para">
A second use of backslash provides a way of encoding
non-printing characters in patterns in a visible manner. There
is no restriction on the appearance of non-printing characters,
apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is
usually easier to use one of the following escape sequences
than the binary character it represents:
</p>
<p class="para">
</p><dl>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\a</em></span>
</dt><dd class="listitem">
<span class="simpara">alarm, that is, the BEL character (hex 07)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\cx</em></span>
</dt><dd class="listitem">
<span class="simpara">"control-x", where x is any character</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\e</em></span>
</dt><dd class="listitem">
<span class="simpara">escape (hex 1B)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\f</em></span>
</dt><dd class="listitem">
<span class="simpara">formfeed (hex 0C)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\n</em></span>
</dt><dd class="listitem">
<span class="simpara">newline (hex 0A)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\r</em></span>
</dt><dd class="listitem">
<span class="simpara">carriage return (hex 0D)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\t</em></span>
</dt><dd class="listitem">
<span class="simpara">tab (hex 09)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\xhh</em></span>
</dt><dd class="listitem">
<span class="simpara">character with hex code hh</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\ddd</em></span>
</dt><dd class="listitem">
<span class="simpara">character with octal code ddd, or backreference</span></dd>
</dl>
<p>
</p>
<p class="para">
The precise effect of "<i>\cx</i>" is as follows:
if "<i>x</i>" is a lower case letter, it is converted
to upper case. Then bit 6 of the character (hex 40) is inverted.
Thus "<i>\cz</i>" becomes hex 1A, but
"<i>\c{</i>" becomes hex 3B, while "<i>\c;</i>"
becomes hex 7B.
</p>
<p class="para">
After "<i>\x</i>", up to two hexadecimal digits are
read (letters can be in upper or lower case).
In <em class="emphasis">UTF-8 mode</em>, "<i>\x{...}</i>" 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 "<i>\0</i>" 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 "<i>\0\x\07</i>"
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 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">
</p><dl>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\040</em></span>
</dt><dd class="listitem">
<span class="simpara">is another way of writing a space</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\40</em></span>
</dt><dd class="listitem">
<span class="simpara">
is the same, provided there are fewer than 40
previous capturing subpatterns
</span>
</dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\7</em></span>
</dt><dd class="listitem">
<span class="simpara">is always a back reference</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\11</em></span>
</dt><dd class="listitem">
<span class="simpara">
might be a back reference, or another way of
writing a tab
</span>
</dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\011</em></span>
</dt><dd class="listitem">
<span class="simpara">is always a tab</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\0113</em></span>
</dt><dd class="listitem">
<span class="simpara">is a tab followed by the character "3"</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\113</em></span>
</dt><dd class="listitem">
<span class="simpara">
is the character with octal code 113 (since there
can be no more than 99 back references)
</span>
</dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\377</em></span>
</dt><dd class="listitem">
<span class="simpara">is a byte consisting entirely of 1 bits</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\81</em></span>
</dt><dd class="listitem">
<span class="simpara">
is either a back reference, or a binary zero
followed by the two characters "8" and "1"
</span>
</dd>
</dl>
<p>
</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 "<i>\b</i>"
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">
</p><dl>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\d</em></span>
</dt><dd class="listitem">
<span class="simpara">any decimal digit</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\D</em></span>
</dt><dd class="listitem">
<span class="simpara">any character that is not a decimal digit</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\h</em></span>
</dt><dd class="listitem">
<span class="simpara">any horizontal whitespace character (since PHP 5.2.4)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\H</em></span>
</dt><dd class="listitem">
<span class="simpara">any character that is not a horizontal whitespace character (since PHP 5.2.4)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\s</em></span>
</dt><dd class="listitem">
<span class="simpara">any whitespace character</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\S</em></span>
</dt><dd class="listitem">
<span class="simpara">any character that is not a whitespace character</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\v</em></span>
</dt><dd class="listitem">
<span class="simpara">any vertical whitespace character (since PHP 5.2.4)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\V</em></span>
</dt><dd class="listitem">
<span class="simpara">any character that is not a vertical whitespace character (since PHP 5.2.4)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\w</em></span>
</dt><dd class="listitem">
<span class="simpara">any "word" character</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\W</em></span>
</dt><dd class="listitem">
<span class="simpara">any "non-word" character</span></dd>
</dl>
<p>
</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 "word" character is any letter or digit or the underscore
character, that is, any character which can be part of a
Perl "<em class="emphasis">word</em>". The definition of letters and digits is
controlled by PCRE's character tables, and may vary if locale-specific
matching is taking place. For example, in the "fr" (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">
</p><dl>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\b</em></span>
</dt><dd class="listitem">
<span class="simpara">word boundary</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\B</em></span>
</dt><dd class="listitem">
<span class="simpara">not a word boundary</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\A</em></span>
</dt><dd class="listitem">
<span class="simpara">start of subject (independent of multiline mode)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\Z</em></span>
</dt><dd class="listitem">
<span class="simpara">
end of subject or newline at end (independent of
multiline mode)
</span>
</dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\z</em></span>
</dt><dd class="listitem">
<span class="simpara">end of subject (independent of multiline mode)</span></dd>
<dt class="varlistentry">
<span class="term"><em class="emphasis">\G</em></span>
</dt><dd class="listitem">
<span class="simpara">first matching position in subject</span></dd>
</dl>
<p>
</p>
<p class="para">
These assertions may not appear in character classes (but
note that "<i>\b</i>" has a different meaning, namely the backspace
character, inside a character class).
</p>
<p class="para">
A word boundary is a position in the subject string where
the current character and the previous character do not both
match <i>\w</i> or <i>\W</i> (i.e. one matches
<i>\w</i> and the other matches
<i>\W</i>), or the start or end of the string if the first
or last character matches <i>\w</i>, respectively.
</p>
<p class="para">
The <i>\A</i>, <i>\Z</i>, and
<i>\z</i> assertions differ from the traditional
circumflex and dollar (described below) in that they only
ever match at the very start and end of the subject string,
whatever options are set. They are not affected by the
<a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_MULTILINE</a> or
<a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_DOLLAR_ENDONLY</a>
options. The difference between <i>\Z</i> and
<i>\z</i> is that <i>\Z</i> matches before a
newline that is the last character of the string as well as at the end of
the string, whereas <i>\z</i> matches only at the end.
</p>
<p class="para">
The <i>\G</i> assertion is true only when the current
matching position is at the start point of the match, as specified by
the <i><tt class="parameter">offset</tt></i>
argument of
<a href="function.preg-match.php" class="function">preg_match()</a>. It differs from <i>\A</i>
when the value of <i><tt class="parameter">offset</tt></i>
is non-zero.
It is available since PHP 4.3.3.
</p>
<p class="para">
<i>\Q</i> and <i>\E</i> can be used to ignore
regexp metacharacters in the pattern since PHP 4.3.3. For example:
<i>\w+\Q.$.\E$</i> will match one or more word characters,
followed by literals <i>.$.</i> and anchored at the end of
the string.
</p>
<p class="para">
<i>\K</i> can be used to reset the match start since
PHP 5.2.4. For example, the pattern <i>foo\Kbar</i> matches
"foobar", but reports that it has matched "bar". The use of
<i>\K</i> does not interfere with the setting of captured
substrings. For example, when the pattern <i>(foo)\Kbar</i>
matches "foobar", the first substring is still set to "foo".
</p>
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