$re = '/\\\\plain(?:(?!\\\\plain).)*\\\\field{\\\\\*\\\\fldinst{ HYPERLINK "(.*?)" }}{\\\\fldrslt{(.*?)}}}}\\\\plain.*? /m';
$str = '\\plain\\f2 This is the first part of the note. This is the second part of the note. This is the \\plain\\f2\\fs24\\cf6{\\txfielddef{\\*\\txfieldstart\\txfieldtype1\\txfieldflags144\\txfielddataval44334\\txfielddata 35003800380039000000}{\\*\\txfielddatadef\\txfielddatatype1\\txfielddata 340034003300330034000000}{\\*\\txfieldtext 20{\\*\\txfieldend}}{\\field{\\*\\fldinst{ HYPERLINK "44334" }}{\\fldrslt{20}}}}\\plain\\f2\\fs24 part of the note.
\\plain\\f2 This is the first part of the note. This is the second part of the note. This is the \\plain\\f2\\fs24\\cf6{\\txfielddef{\\*\\txfieldstart\\txfieldtype1\\txfieldflags144\\txfielddataval44334\\txfielddata 35003800380039000000}{\\*\\txfielddatadef\\txfielddatatype1\\txfielddata 340034003300330034000000}{\\*\\txfieldtext 20{\\*\\txfieldend}}{\\field{\\*\\fldinst{ HYPERLINK "44334" }}{\\fldrslt{20}}}}\\plain\\f2\\fs24';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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