$re = '/(?<=src=")(.*?)\s+(.*?)(?=\.)/x';
$str = 'text here
<img src="foo_image.gif" height="42" width="42">
text here
<img height="42" width="42" src="another_foo image.png" height="42" width="42">
text here
<img src="/path/to/yetanother foo image.jpg" height="42" width="42">
text here';
$subst = "$1_$2";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php