获得唯一途径

str_replace传递要删除的元素数组可以轻松工作：

$inputString = '<img src="images/stories/2017/12/2-51.jpg">';

$cuttingData = array('<img src="', '">');
$result = str_replace($cuttingData, '', $inputString);

echo $result;

使用简单的 HTML DOM 解析器：

$html = str_get_html('<img src="images/stories/2017/12/2-51.jpg">');

// Find all images
foreach($html->find("img") as $element) {
    echo $element->src , "<br>"; // выведет images/stories/2017/12/2-51.jpg
}

你可以看看http://phpfiddle.org/main/code/jkmb-miie

$html = str_get_html('<div><img src="images/stories/2017/12/2-51.jpg"><img src="images/stories/2015/10/2-52.jpg"><img src="images/stories/2016/12/2-53.jpg"></div>');

// Find all images
foreach($html->find("img") as $element) {
    echo $element->src , "<br>";
}

// Выведет 
images/stories/2017/12/2-51.jpg
images/stories/2015/10/2-52.jpg
images/stories/2016/12/2-53.jpg

你可以看看http://phpfiddle.org/main/code/iws5-2vex

使用DOMDocument（已经在PHP中原生提供）

$dom = new DOMDocument;
$dom->loadHTML('<img src="images/stories/2017/12/2-51.jpg">');
$images = $dom->getElementsByTagName('img');

foreach ($images as $image) {
    echo $image->getAttribute('src').'<br/>';
}

-

$dom = new DOMDocument;
$dom->loadHTML('<div><img src="images/stories/2017/12/2-51.jpg"><img src="images/stories/2015/10/2-52.jpg"><img src="images/stories/2016/12/2-53.jpg"></div>');
$images = $dom->getElementsByTagName('img');

foreach ($images as $image) {
    echo $image->getAttribute('src').'<br/>';
}

// Выведет 
images/stories/2017/12/2-51.jpg
images/stories/2015/10/2-52.jpg
images/stories/2016/12/2-53.jpg

作为这样的选择：

$html = '<img src="images/stories/2017/12/2-51.jpg">';
$src = preg_replace('/.*<img[^>]+src="([^"]+)".*/', '$1', $html);
echo $src;

或使用preg_match_all()

$str = '<img src="images/stories/2017/12/1-51.jpg"> текст <img src="images/stories/2017/12/2-51.jpg">';
preg_match_all('~(?<=src=").+?(?=")~', $str, $a);

var_dump($a[0]);