ch04_06.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

</table><p>

<p><a name="INDEX-619" /><a name="INDEX-620" />The <tt class="literal">.</tt> (single
dot) is a wildcard character. When used in a regular expression, it
can match any single character. The exception is the newline
character (<tt class="literal">\n</tt>), except when you use the
<tt class="literal">/s</tt> modifier on the pattern match operator. This
modifier treats the string to be matched against as a single
"long" string with embedded
newlines.
</p>

<p><a name="INDEX-621" /><a name="INDEX-622" /><a name="INDEX-623" /><a name="INDEX-624" /><a name="INDEX-625" />The
<tt class="literal">^</tt> and <tt class="literal">$</tt> metacharacters are used
as anchors in a regular expression. The <tt class="literal">^</tt> matches
the beginning of a line. This character should appear only at the
beginning of an expression to match the beginning of the line. The
exception to this is when the <tt class="literal">/m</tt> (multiline)
modifier is used, in which case it will match at the beginning of the
string and after every newline (except the last, if there is one).
Otherwise, <tt class="literal">^</tt> will match itself, unescaped,
anywhere in a pattern, except if it is the first character in a
bracketed character class, in which case it negates the class.
</p>

<p>Similarly, <tt class="literal">$</tt> will match the end of a line (just
before a newline character) only if it is at the end of a pattern,
unless <tt class="literal">/m</tt> is used, in which case it matches just
before every newline and at the end of a string. You need to escape
<tt class="literal">$</tt> to match a literal dollar sign in all cases,
because if <tt class="literal">$</tt> isn't at the end of
a pattern (or placed right before a <tt class="literal">)</tt> or
<tt class="literal">]</tt>), Perl will attempt to do variable
interpretation. The same holds true for the <tt class="literal">@</tt>
sign, which Perl will interpret as an array variable start unless it
is backslashed.
</p>

<p><a name="INDEX-626" /><a name="INDEX-627" /><a name="INDEX-628" /><a name="INDEX-629" /><a name="INDEX-630" /><a name="INDEX-631" />The <tt class="literal">*</tt>,
<tt class="literal">+</tt>, and <tt class="literal">?</tt> metacharacters are
called <em class="emphasis">quantifiers</em>. They specify the number of
times to match something. They act on the element immediately
preceding them, which could be a single character (including the
<tt class="literal">.</tt>), a grouped expression in parentheses, or a
character class. The <tt class="literal">{...}</tt> construct is a
generalized modifier. You can put two numbers separated by a comma
within the braces to specify minimum and maximum numbers that the
preceding element can match.
</p>

<p><a name="INDEX-632" /><a name="INDEX-633" />Parentheses are used
to group characters or expressions. They also have the side effect of
remembering what they matched so you can recall and reuse patterns
with a special group of variables.
</p>

<p><a name="INDEX-634" /><a name="INDEX-635" />The
<tt class="literal">|</tt> is the alternation operator in regular
expressions. It matches either what's on its left
side or right side. It does not affect only single characters. For
example:
</p>

<blockquote><pre class="code">/you|me|him|her/</pre></blockquote>

<p>looks for any of the four words. You should use parentheses to
provide boundaries for alternation:
</p>

<blockquote><pre class="code">/And(y|rew)/</pre></blockquote>

<p>This will match either "Andy" or
"Andrew".
</p>

</div>
<a name="perlnut2-CHP-4-SECT-6.3" /><div class="sect2">
<h3 class="sect2">4.6.3. Escaped Sequences</h3>

<p><a name="INDEX-636" /><a name="INDEX-637" /><a name="INDEX-638" /><a name="INDEX-639" /><a name="INDEX-640" /><a name="INDEX-641" /><a name="INDEX-642" /><a name="INDEX-643" /><a name="INDEX-644" /><a name="INDEX-645" /><a name="INDEX-646" /><a name="INDEX-647" />The
following table lists the backslashed representations of characters
that you can use in regular expressions: <a name="INDEX-648" /><a name="INDEX-649" />
</p>

<a name="ch04-10-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>Code</p>
</th>
<th>
<p>Matches</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">\a</tt></p>
</td>
<td>
<p>Alarm (beep)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\n</tt></p>
</td>
<td>
<p>Newline</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\r</tt></p>
</td>
<td>
<p>Carriage return</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\t</tt></p>
</td>
<td>
<p>Tab</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\f</tt></p>
</td>
<td>
<p>Formfeed</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\e</tt></p>
</td>
<td>
<p>Escape</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\038</tt></p>
</td>
<td>
<p>Any octal ASCII value</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\x7f</tt></p>
</td>
<td>
<p>Any hexadecimal ASCII value</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\x{263a}</tt></p>
</td>
<td>
<p>A wide hexadecimal value</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\c</tt><em class="replaceable"><tt>x</tt></em></p>
</td>
<td>
<p>Control-<em class="replaceable"><tt>x</tt></em></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\N{</tt><em class="replaceable"><tt>name</tt></em><tt class="literal">}</tt>
</p>
</td>
<td>
<p>A named character</p>
</td>
</tr>

</table><p>

</div>
<a name="perlnut2-CHP-4-SECT-6.4" /><div class="sect2">
<h3 class="sect2">4.6.4. Character Classes</h3>

<p><a name="INDEX-650" /><a name="INDEX-651" /><a name="INDEX-652" /><a name="INDEX-653" /><a name="INDEX-654" /><a name="INDEX-655" /><a name="INDEX-656" />The <tt class="literal">[...]</tt> construct
is used to list a set of characters (a <em class="emphasis">character
class</em>) of which <em class="emphasis">one</em> will match.
Brackets are often used when capitalization is uncertain in a match:
</p>

<blockquote><pre class="code">/[tT]here/</pre></blockquote>

<p><a name="INDEX-657" /><a name="INDEX-658" /><a name="INDEX-659" />A dash (<tt class="literal">-</tt>) may be
used to indicate a range of characters in a character class:
</p>

<blockquote><pre class="code">/[a-zA-Z]/;  # Match any single letter
/[0-9]/;     # Match any single digit</pre></blockquote>

<p>To put a literal dash in the list you must use a backslash before it
(<tt class="literal">\-</tt>).
</p>

<p><a name="INDEX-660" /><a name="INDEX-661" />By placing a <tt class="literal">^</tt> as
the first element in the brackets, you create a negated character
class, i.e., it matches any character not in the list. For example:
</p>

<blockquote><pre class="code">/[^A-Z]/;    # Matches any character other than an uppercase letter</pre></blockquote>

<p>Some common character classes have their own predefined escape
sequences for your programming
convenience<a name="INDEX-662" /><a name="INDEX-663" /><a name="INDEX-664" /><a name="INDEX-665" /><a name="INDEX-666" /><a name="INDEX-667" /><a name="INDEX-668" /><a name="INDEX-669" /><a name="INDEX-670" /><a name="INDEX-671" />:
</p>

<a name="ch04-11-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>Code</p>
</th>
<th>
<p>Matches</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">\d</tt></p>
</td>
<td>
<p>A digit, same as <tt class="literal">[0-9]</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\D</tt></p>
</td>
<td>
<p>A nondigit, same as <tt class="literal">[^0-9]</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\w</tt></p>
</td>
<td>
<p>A word character (alphanumeric), same as
<tt class="literal">[a-zA-Z_0-9]</tt> 
</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\W</tt></p>
</td>
<td>
<p>A non-word character, <tt class="literal">[^a-zA-Z_0-9]</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\s</tt></p>
</td>
<td>
<p>A whitespace character, same as <tt class="literal">[ \t\n\r\f]</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\S</tt></p>
</td>
<td>
<p>A non-whitespace character, <tt class="literal">[^ \t\n\r\f]</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\C</tt></p>
</td>
<td>
<p>Match a character (byte)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\pP</tt></p>
</td>
<td>
<p>Match P-named (Unicode) property</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\PP</tt></p>
</td>
<td>
<p>Match non-P</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\X</tt></p>
</td>
<td>
<p>Match extended unicode sequence</p>
</td>
</tr>

</table><p>

<p>While Perl implements <tt class="literal">lc()</tt> and <tt class="literal">uc(
)</tt>, which you can use for testing the proper case of words
or characters, you can do the same with escape
sequences<a name="INDEX-672" /><a name="INDEX-673" /><a name="INDEX-674" /><a name="INDEX-675" /><a name="INDEX-676" /><a name="INDEX-677" />:
</p>

<a name="ch04-12-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>Code</p>
</th>
<th>
<p>Matches</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">\l</tt></p>
</td>
<td>
<p>Lowercase until next character</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\u</tt></p>
</td>
<td>
<p>Uppercase until next character</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\L</tt></p>
</td>
<td>
<p>Lowercase until <tt class="literal">\E</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\U</tt></p>
</td>
<td>
<p>Uppercase until <tt class="literal">\E</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\Q</tt></p>
</td>
<td>
<p>Disable pattern metacharacters until <tt class="literal">\E</tt></p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\E</tt></p>
</td>
<td>
<p>End case modification</p>
</td>
</tr>

</table><p>

<p>These elements match any single element in (or not in) their class. A
<tt class="literal">\w</tt> matches only one character of a word. Using a
modifier, you can match a whole word, for example, with
<tt class="literal">\w+</tt>. The abbreviated classes may also be used
within brackets as elements of other character classes.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-6.5" /><div class="sect2">
<h3 class="sect2">4.6.5. Anchors</h3>

<p><a name="INDEX-678" /><a name="INDEX-679" /><a name="INDEX-680" /><a name="INDEX-681" />Anchors
don't match any characters; they match places within
a string. The two most common anchors are <tt class="literal">^</tt> and
<tt class="literal">$</tt>, which match the beginning and end of a line,
respectively. The following table lists the anchoring patterns used
to match certain boundaries in regular expressions:
</p>

<a name="ch04-13-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>Assertion</p>
</th>
<th>
<p>Meaning</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">^</tt></p>
</td>
<td>
<p>Matches at the beginning of the string (or line, if
<tt class="literal">/m</tt> is used)
</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">$</tt></p>