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<h2 class="title">Subpatterns</h2>
<p class="para">
Subpatterns are delimited by parentheses (round brackets),
which can be nested. Marking part of a pattern as a subpattern
does two things:
</p>
<ol type="1">
<li class="listitem">
<p class="para">
It localizes a set of alternatives. For example, the pattern
<em>cat(aract|erpillar|)</em> matches one of the words "cat",
"cataract", or "caterpillar". Without the parentheses, it would match
"cataract", "erpillar" or the empty string.
</p>
</li>
<li class="listitem">
<p class="para">
It sets up the subpattern as a capturing subpattern (as defined above).
When the whole pattern matches, that portion of the subject string
that matched the subpattern is passed back to the caller via the
<em class="emphasis">ovector</em> argument of <span class="function"><strong>pcre_exec()</strong></span>.
Opening parentheses are counted from left to right (starting from 1) to
obtain the numbers of the capturing subpatterns.
</p>
</li>
</ol>
<p class="para">
For example, if the string "the red king" is matched against
the pattern
<em>the ((red|white) (king|queen))</em>
the captured substrings are "red king", "red", and "king",
and are numbered 1, 2, and 3.
</p>
<p class="para">
The fact that plain parentheses fulfill two functions is not
always helpful. There are often times when a grouping subpattern
is required without a capturing requirement. If an
opening parenthesis is followed by "?:", the subpattern does
not do any capturing, and is not counted when computing the
number of any subsequent capturing subpatterns. For example,
if the string "the white queen" is matched against the
pattern
<em>the ((?:red|white) (king|queen))</em>
the captured substrings are "white queen" and "queen", and
are numbered 1 and 2. The maximum number of captured substrings
is 99, and the maximum number of all subpatterns,
both capturing and non-capturing, is 200.
</p>
<p class="para">
As a convenient shorthand, if any option settings are
required at the start of a non-capturing subpattern, the
option letters may appear between the "?" and the ":". Thus
the two patterns
</p>
<div class="informalexample">
<div class="example-contents">
<div class="cdata"><pre>
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
</pre></div>
</div>
</div>
<p class="para">
match exactly the same set of strings. Because alternative
branches are tried from left to right, and options are not
reset until the end of the subpattern is reached, an option
setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".
</p>
<p class="para">
It is possible to name a subpattern using the syntax
<em>(?P<name>pattern)</em>. This subpattern will then
be indexed in the matches array by its normal numeric position and
also by name. PHP 5.2.2 introduced two alternative syntaxes
<em>(?<name>pattern)</em> and <em>(?'name'pattern)</em>.
</p>
<p class="para">
Sometimes it is necessary to have multiple matching, but alternating
subgroups in a regular expression. Normally, each of these would be given
their own backreference number even though only one of them would ever
possibly match. To overcome this, the <em>(?|</em> syntax allows
having duplicate numbers. Consider the following regex matched against the
string <em>Sunday</em>:
</p>
<div class="informalexample">
<div class="example-contents">
<div class="cdata"><pre>(?:(Sat)ur|(Sun))day</pre></div>
</div>
</div>
<p class="para">
Here <em>Sun</em> is stored in backreference 2, while
backreference 1 is empty. Matching yields <em>Sat</em> in
backreference 1 while backreference 2 does not exist. Changing the pattern
to use the <em>(?|</em> fixes this problem:
</p>
<div class="informalexample">
<div class="example-contents">
<div class="cdata"><pre>(?|(Sat)ur|(Sun))day</pre></div>
</div>
</div>
<p class="para">
Using this pattern, both <em>Sun</em> and <em>Sat</em>
would be stored in backreference 1.
</p>
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