regex.php

<?php
$str = '<div id = "content"><div>123</div></div><div>123</div>';
$re = '% # Match a DIV element having id="content".
    <div\b             # Start of outer DIV start tag.
    [^>]*?             # Lazily match up to id attrib.
    \bid\s*+=\s*+      # id attribute name and =
    ([\'"]?+)          # $1: Optional quote delimiter.
    \bfooter\b        # specific ID to be matched.
    (?(1)\1)           # If open quote, match same closing quote
    [^>]*+>            # remaining outer DIV start tag.
    (                  # $2: DIV contents. (may be called recursively!)
      (?:              # Non-capture group for DIV contents alternatives.
      # DIV contents option 1: All non-DIV, non-comment stuff...
        [^<]++         # One or more non-tag, non-comment characters.
      # DIV contents option 2: Start of a non-DIV tag...
      | <            # Match a "<", but only if it
        (?!          # is not the beginning of either
          /?div\b    # a DIV start or end tag,
        | !--        # or an HTML comment.
        )            # Ok, that < was not a DIV or comment.
      # DIV contents Option 3: an HTML comment.
      | <!--.*?-->     # A non-SGML compliant HTML comment.
      # DIV contents Option 4: a nested DIV element!
      | <div\b[^>]*+>  # Inner DIV element start tag.
        (?2)           # Recurse group 2 as a nested subroutine.    
        </div\s*>      # Inner DIV element end tag.
      )*+              # Zero or more of these contents alternatives.
    )                  # End 2$: DIV contents.
    </div\s*>          # Outer DIV end tag.
    %isx';
if (preg_match($re, $str, $matches)) {
    printf("Match found:\n%s\n", $matches[0]);
}