ch04_06.htm

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

</td>
<td>
<p>Matches at the end of the string (or line, if <tt class="literal">/m</tt>
is used)
</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\b</tt></p>
</td>
<td>
<p>Matches at word boundary (between <tt class="literal">\w</tt> and
<tt class="literal">\W</tt>)
</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\B</tt></p>
</td>
<td>
<p>Matches except at word boundary</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\A</tt></p>
</td>
<td>
<p>Matches at the beginning of the string</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\Z</tt></p>
</td>
<td>
<p>Matches at the end of the string or before a newline</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\z</tt></p>
</td>
<td>
<p>Matches only at the end of the string</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\G</tt></p>
</td>
<td>
<p>Matches where previous <tt class="literal">m//g</tt> left off</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">\c</tt></p>
</td>
<td>
<p>Suppresses resetting of search position when used with \g. Without
\c, search pattern is reset to the beginning of the string.
</p>
</td>
</tr>

</table><p>

<p>The <tt class="literal">$</tt> and <tt class="literal">\Z</tt> assertions can
match not only at the end of the string, but also one character
earlier than that, if the last character of the string is a newline.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-6.6" /><div class="sect2">
<h3 class="sect2">4.6.6. Quantifiers</h3>

<p><a name="INDEX-682" /><a name="INDEX-683" /><a name="INDEX-684" /><a name="INDEX-685" />Quantifiers
are used to specify the number of instances of the previous element
that can match. For instance, you could say "match
any number of a's, including none"
(<tt class="literal">a*</tt>), or "match between 5 and 10
instances of the word 'owie'
(<tt class="literal">(owie){5,10}</tt>)". 
</p>

<p><a name="INDEX-686" />Quantifiers,
by nature, are greedy. That is, the way the Perl regular expression
"engine" works is that it will look
for the biggest match possible (the farthest to the right) unless you
tell it not to. Say you are searching a string that reads:
</p>

<blockquote><pre class="code">a whatever foo, b whatever foo</pre></blockquote>

<p>and you want to find <tt class="literal">a</tt> and <tt class="literal">foo</tt>
with something in between. You might use:
</p>

<blockquote><pre class="code">/a.*foo/</pre></blockquote>

<p>A <tt class="literal">.</tt> followed by a <tt class="literal">*</tt> looks for
any character, any number of times, until <tt class="literal">foo</tt> is
found. But since Perl will look as far to the right as possible to
find <tt class="literal">foo</tt>, the first instance of
<tt class="literal">foo</tt> is swallowed up by the greedy
<tt class="literal">.*</tt> expression.
</p>

<p><a name="INDEX-687" /><a name="INDEX-688" /><a name="INDEX-689" />Therefore,
a<a name="INDEX-690" />ll the quantifiers have a notation
that allows for minimal matching, so they are nongreedy. This
notation uses a question mark immediately following the quantifier to
force Perl to look for the earliest available match (farthest to the
left). The following table lists the regular expression quantifiers
and their nongreedy forms:
</p>

<a name="ch04-14-fm2xml" /><table border="1" cellpadding="3">




<tr>
<th>
<p>Maximal</p>
</th>
<th>
<p>Minimal</p>
</th>
<th>
<p>Allowed range</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">{n</tt>,<tt class="literal">m}</tt></p>
</td>
<td>
<p><tt class="literal">{n</tt>,<tt class="literal">m}?</tt></p>
</td>
<td>
<p>Must occur at least <tt class="literal">n</tt> times but no more than
<tt class="literal">m</tt> times
</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">{n,}</tt></p>
</td>
<td>
<p><tt class="literal">{n,}?</tt></p>
</td>
<td>
<p>Must occur at least <tt class="literal">n</tt> times</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">{n}</tt></p>
</td>
<td>
<p><tt class="literal">{n}?</tt></p>
</td>
<td>
<p>Must match exactly <tt class="literal">n</tt> times</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">*</tt></p>
</td>
<td>
<p><tt class="literal">*?</tt></p>
</td>
<td>
<p>0 or more times (same as <tt class="literal">{0,}</tt>)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">+</tt></p>
</td>
<td>
<p><tt class="literal">+?</tt></p>
</td>
<td>
<p>1 or more times (same as <tt class="literal">{1,}</tt>)</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">?</tt></p>
</td>
<td>
<p><tt class="literal">??</tt></p>
</td>
<td>
<p>0 or 1 time (same as <tt class="literal">{0,1}</tt>)</p>
</td>
</tr>

</table><p>

</div>
<a name="perlnut2-CHP-4-SECT-6.7" /><div class="sect2">
<h3 class="sect2">4.6.7. Pattern Match Variables</h3>

<p><a name="INDEX-691" /><a name="INDEX-692" />Parentheses not only group elements
in a regular expression, they also remember the patterns they match.
Every match from a parenthesized element is saved to a special,
read-only variable indicated by a number. You can recall and reuse a
match by using these variables.
</p>

<p><a name="INDEX-693" /><a name="INDEX-694" /><a name="INDEX-695" />Within a pattern, each
parenthesized element saves its match to a numbered variable, in
order starting with <tt class="literal">1</tt>. You can recall these
matches within the expression by using <tt class="literal">\1</tt>,
<tt class="literal">\2</tt>, and so on.
</p>

<p>Outside of the matching pattern, the matched variables are recalled
with the usual dollar sign, i.e., <tt class="literal">$1</tt>,
<tt class="literal">$2</tt>, etc. The dollar sign notation should be used
in the replacement expression of a substitution and anywhere else you
might want to use the variables in your program. For example, to
implement "i before e, except after
c":
</p>

<blockquote><pre class="code">s/([^c])ei/$1ie/g;</pre></blockquote>

<p>The backreferencing variables are:</p>

<dl>
<dt><b><tt class="literal">$+</tt></b></dt>
<dd>
Returns the last parenthesized pattern match</p>
</dd>


<dt><b><tt class="literal">$&amp;</tt></b></dt>
<dd>
Returns the entire matched string</p>
</dd>


<dt><b><tt class="literal">$'</tt></b></dt>
<dd>
Returns everything before the matched string</p>
</dd>


<dt><b><tt class="literal">$'</tt></b></dt>
<dd>
Returns everything after the matched string</p>
</dd>

</dl>

<p>Backreferencing with these variables will slow down your program
noticeably for all regular expressions.
</p>

</div>
<a name="perlnut2-CHP-4-SECT-6.8" /><div class="sect2">
<h3 class="sect2">4.6.8. Extended Regular Expressions</h3>

<p><a name="INDEX-696" /><a name="INDEX-697" /><a name="INDEX-698" /><a name="INDEX-699" />Perl defines an
extended syntax for regular expressions. The syntax is a pair of
parentheses with a question mark as the first thing within the
parentheses. The character after the question mark gives the function
of the extension. The extensions are:
</p>

<dl>
<dt><b><tt class="literal">(?#</tt><em class="replaceable">text</em><tt class="literal">)</tt></b></dt>
<dd>
A comment. The text is ignored.</p>
</dd>


<dt><b><tt class="literal">(?:...)</tt></b></dt>
<dt><b><tt class="literal">(?imsx-imsx:...)</tt></b></dt>
<dd>
This groups things like <tt class="literal">(...)</tt> but
doesn't make backreferences.
</p>
</dd>


<dt><b><tt class="literal">(?=...)</tt></b></dt>
<dd>
A zero-width positive lookahead assertion. For example,
<tt class="literal">/\w+(?=\t)/</tt> matches a word followed by a tab,
without including the tab in <tt class="literal">$&amp;</tt>.
</p>
</dd>


<dt><b><tt class="literal">(?!...)</tt></b></dt>
<dd>
A zero-width negative lookahead assertion. For example,
<tt class="literal">/foo(?!bar)/</tt> matches any occurrence of
<tt class="literal">foo</tt> that isn't followed by
<tt class="literal">bar</tt>.
</p>
</dd>


<dt><b><tt class="literal">(?&lt;...)</tt></b></dt>
<dd>
A zero-width positive lookbehind assertion. For example,
<tt class="literal">/(?&lt;bad)boy/</tt> matches the word
<tt class="literal">boy</tt> that follows <tt class="literal">bad</tt>, without
including <tt class="literal">bad</tt> in <tt class="literal">$&amp;</tt>. This
works only for fixed-width lookbehind.
</p>
</dd>


<dt><b><tt class="literal">(?{</tt><em class="replaceable">code</em><tt class="literal">})</tt></b></dt>
<dd>
An experimental regular expression feature to evaluate any embedded
Perl code. This evaluation always succeeds, and
<em class="replaceable"><tt>code</tt></em> is not interpolated.
</p>
</dd>


<dt><b><tt class="literal">(?&lt;!=...)</tt></b></dt>
<dd>
A zero-width negative lookbehind assertion. For example,
<tt class="literal">/(?&lt;!=bad)boy/</tt> matches any occurrence of
<tt class="literal">boy</tt> that doesn't follow
<tt class="literal">bad</tt>. This works only for fixed-width lookbehind.
</p>
</dd>


<dt><b><tt class="literal">(?&gt;...)</tt></b></dt>
<dd>
Matches the substring that the standalone pattern would match if
anchored at the given position.
</p>
</dd>


<dt><b><tt class="literal">(?(</tt><em class="replaceable">condition</em><tt class="literal">)</tt><em class="replaceable">yes-pattern</em>|<em class="replaceable">no-pattern</em><tt class="literal">)</tt></b></dt>
<dt><b><tt class="literal">(?(</tt><em class="replaceable">condition</em>)<em class="replaceable">yes-pattern</em><tt class="literal">)</tt></b></dt>
<dd>
Matches a pattern determined by a condition.
<em class="replaceable"><tt>condition</tt></em> should be either an integer,
which is true if the pair of parentheses corresponding to the integer
has matched, or a lookahead, lookbehind, or evaluate, zero-width
assertion. <em class="replaceable"><tt>no-pattern</tt></em> will be used to
match if the condition was not meant, but it is also optional.
</p>
</dd>


<dt><b><tt class="literal">(?imsx-imsx)</tt></b></dt>
<dd>
One or more embedded pattern-match modifiers. Modifiers are switched
off if they follow a <tt class="literal">-</tt> (dash). The modifiers are
defined as follows<a name="INDEX-700" /><a name="INDEX-701" /><a name="INDEX-702" /><a name="INDEX-703" />:
</p>


<a name="ch04-15-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>Modifier</p>
</th>
<th>
<p>Meaning</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">i</tt></p>
</td>
<td>
<p>Do case-insensitive pattern matching.</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">m</tt></p>
</td>
<td>
<p>Treat string as multiple lines.</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">s</tt></p>
</td>
<td>
<p>Treat string as single line.</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">x</tt></p>
</td>
<td>
<p>Use extended regular expressions.</p>
</td>
</tr>

</table><p>
</dd>

</dl>
</div>


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