$re = '/<item (.*)(Misc\/)(\d*)_(\w*)_(\d*)(.*)(\w*) id="([^*])(\d*)(.+)_(\d+)(\w*)(.*")(.*=")(\w*)(.*)" \/>/i';
$str = '<item href="Misc/9781620025420_epub3_001.smil" id="x9781620025420_epub3_001.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_001.smil" id="x9781620025420_epub3_001.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_002.smil" id="x9781620025420_epub3_002.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_003.smil" id="x9781620025420_epub3_003.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_004.smil" id="x9781620025420_epub3_004.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_005.smil" id="x9781620025420_epub3_005.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_006.smil" id="x9781620025420_epub3_006.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_007.smil" id="x9781620025420_epub3_007.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_008.smil" id="x9781620025420_epub3_008.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_009.smil" id="x9781620025420_epub3_009.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_010.smil" id="x9781620025420_epub3_010.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_011.smil" id="x9781620025420_epub3_011.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_012.smil" id="x9781620025420_epub3_012.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_013.smil" id="x9781620025420_epub3_013.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_014.smil" id="x9781620025420_epub3_014.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_015.smil" id="x9781620025420_epub3_015.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_016.smil" id="x9781620025420_epub3_016.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_017.smil" id="x9781620025420_epub3_017.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_018.smil" id="x9781620025420_epub3_018.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_019.smil" id="x9781620025420_epub3_019.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_020.smil" id="x9781620025420_epub3_020.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_021.smil" id="x9781620025420_epub3_021.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_022.smil" id="x9781620025420_epub3_022.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_023.smil" id="x9781620025420_epub3_023.smil" media-type="text/plain" />
<item href="Misc/9781620025420_epub3_024.smil" id="x9781620025420_epub3_024.smil" media-type="text/plain" />';
$subst = "<item \1\2\3\4_\5.\6 id=\"\7\8\9_\10\11\12\13\14\15\16\17";
$result = preg_replace($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