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    <p>Regex is probably <em>one</em> way:</p> <pre><code>$src = preg_replace('(.*?(?&lt;==)([^&amp;"]+).*)i', '\\1', $text); </code></pre> <p>However I would give the following hint as <em>the way</em>: Divide an conquer. Divide a problem into smaller ones and solve the overall problem step-by-step then. This works for many problems. As an example:</p> <ul> <li>First: Get the SRC attribute value from the string</li> </ul> <p>There are a thousand ways to do this, incl. regular expressions. As a regex would assume the string is always formatted this way and extracting the URL attribute value is actually trivial, I am using a different function that supports regular expressions: <a href="http://php.net/sscanf" rel="nofollow"><code>sscanf</code></a>:</p> <pre><code>$url = sscanf($text, '&lt;iframe src="%[^"]')[0]; # string(126) "http://www.google.com/calendar/embed?src=canaca.../Tokyo" </code></pre> <p>So now the URL is already extracted. As this is an URL, it can be processed with standard URL functions. Let's see:</p> <ul> <li>Second: Parse the query from the URL</li> </ul> <p>To get the SRC value from the URL you could use a regular expression again. However, as PHP has functions that are specific to URL handling, I use those instead. I can exactly say what I need with <a href="http://php.net/parse_url" rel="nofollow"><code>parse_url</code></a>. And this time I first of all need the <em>query</em> part of the URL. That is the part that has the query variables after the question mark:</p> <pre><code>$query = parse_url($url, PHP_URL_QUERY); # string(89) "src=canacad.ac.jp_dqrg6k9pg1s879somecodekj88.../Tokyo" </code></pre> <p>This is already one step further to the value we're looking for. So there is another step to do:</p> <ul> <li>Third: Parse the SRC value from the query</li> </ul> <p>Here again PHP has a function built in to do that. We can extract all variables in a query from an URL with the <a href="http://php.net/parse_str" rel="nofollow"><code>parse_str</code></a> function. As it returns the results via a function parameter, this now needs two lines of code:</p> <pre><code>parse_str($query, $vars); $src = $vars['src']; # string(68) "canacad.ac.jp_dqrg6k9pg1s879somecodekj88c8@group.calendar.google.com" </code></pre> <p>And now in the <code>$src</code> variable is the value you're looking for.</p> <p>Here the whole code from above at a glance:</p> <pre><code>$text = '&lt;iframe src="http://www.google.com/calendar/embed?src=canacad.ac.jp_dqrg6k9pg1s879somecodekj88c8%40group.calendar.google.com&amp;ctz=Asia/Tokyo" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"&gt;&lt;/iframe&gt;'; $url = sscanf($text, '&lt;iframe src="%[^"]')[0]; $query = parse_url($url, PHP_URL_QUERY); parse_str($query, $vars); $src = $vars['src']; var_dump($url, $query, $src); </code></pre> <p>The output is as follows, showing all three steps:</p> <pre><code>string(126) "http://www.google.com/calendar/embed?src=canacad.ac.jp_dqrg6k9pg1s879somecodekj88c8%40group.calendar.google.com&amp;ctz=Asia/Tokyo" string(89) "src=canacad.ac.jp_dqrg6k9pg1s879somecodekj88c8%40group.calendar.google.com&amp;ctz=Asia/Tokyo" string(68) "canacad.ac.jp_dqrg6k9pg1s879somecodekj88c8@group.calendar.google.com" </code></pre> <p>So regardless which functions you use in each of those steps: if you divide a problem into smaller parts you nearly always will be able to solve larger problems. And also if there is a problem in one of the sub-steps, you only need to fix a single step - not the whole operation. If you use a single regular expression to do all this work, you would have the single point of failure (and crafting a good regular expression in the world of HTML and URLs is non-trivial so it likely will break).</p> <p>A perfect solution would use an HTML parser for the first step for example. For example with the <a href="http://php.net/book.tidy" rel="nofollow"><em>Tidy extension</em></a> or with the popular <em>DOMDocument extension</em>:</p> <pre><code>// Tidy (non error-checked): $url = tidy_parse_string($text)-&gt;body()-&gt;child[0]-&gt;attribute['src']; // DOMDocument (non error-checked): $url = @DOMDocument::loadHTML($text)-&gt;getElementsByTagname('iframe') -&gt;item(0)-&gt;getAttribute('src'); </code></pre> <p>A HTML parser has the benefit that it understand the HTML elements. You can look for specific elements and attributes even if their position changes.</p>
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