$re = '/<((?!a|\/a)[^>]*)>\s*/m';
$str = '<html>
<head>
<meta http-equiv=\\"Content-Type\\" content=\\"text/html; charset=utf-8\\">
<meta content=\\"text/html; charset=us-ascii\\">
<meta name=\\"ProgId\\" content=\\"Word.Document\\">
<meta name=\\"Generator\\" content=\\"Microsoft Word 15\\">
<meta name=\\"Originator\\" content=\\"Microsoft Word 15\\">
<style>
<!--
@font-face
\\t{font-family:\\"Cambria Math\\"}
@font-face
\\t{font-family:Calibri}
p.MsoNormal, li.MsoNormal, div.MsoNormal
\\t{margin:0cm;
\\tmargin-bottom:.0001pt;
\\tfont-size:11.0pt;
\\tfont-family:\\"Calibri\\",sans-serif}
a:link, span.MsoHyperlink
\\t{color:#0563C1;
\\ttext-decoration:underline}
a:visited, span.MsoHyperlinkFollowed
\\t{color:#954F72;
\\ttext-decoration:underline}
p.msonormal0, li.msonormal0, div.msonormal0
\\t{margin-right:0cm;
\\tmargin-left:0cm;
\\tfont-size:11.0pt;
\\tfont-family:\\"Calibri\\",sans-serif}
span.EmailStyle18
\\t{font-family:\\"Calibri\\",sans-serif}
.MsoChpDefault
\\t{font-size:10.0pt;
\\tfont-family:\\"Calibri\\",sans-serif}
@page WordSection1
\\t{margin:72.0pt 72.0pt 72.0pt 72.0pt}
div.WordSection1
\\t{}
-->
</style>
</head>
<body lang=\\"EN-GB\\" link=\\"#0563C1\\" vlink=\\"#954F72\\" style=\\"\\">
<div class=\\"WordSection1\\">
<p class=\\"MsoNormal\\">I NEED TO HAVE THIS STRING</p>
<p class=\\"MsoNormal\\"> </p>
<p class=\\"MsoNormal\\"><span style=\\"\\">AND I NEED THE FOLLOWING ANCHOR
<a href=\\"google.com\\">
THIS ONE</a>.</span></p>
<p class=\\"MsoNormal\\"> </p>
</div>
</body>
</html>
';
$subst = "";
$result = preg_replace($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