Thank you for sharing that code. I hope one of the visitors of this page will find it useful. Regards!

You can define the start and end of a section to search for (in this case ‘<a' and '</a' to find anchors), define a trigger term (in this case 'href=' to find linked anchors), and define exclusion terms (so in the case that the trigger term exists within the section but one of the exclusion terms also exists, in this case either an img '<img' or a relative reference 'www.domain.com', '"#', or '""', the new text will not be inserted.

The new text is defined as $r. The function is not as hairy as it looks — I just commented it up profusely.

Hope this helps someone
C

============================================
function exticon( $t ) {
// $n is the needle array, where
// $n[0] defines the section beginning,
// $n[1] defines the trigger term, or the term that determines insertion,
// $n[2] defines the section end,
// $n[3] – $n[n] define exclusion term/s, or the term/s that preclude insertion

$n = array(
"<a", // section beginning
"href=", // trigger term
"</a", // section end
"<img", // from here to end – exclusion terms
"www.domain.com",
"\"#",
"\"\""
);
// $pN is a position marker to move the point of focus along the string
$pN = 0;
// $r is the text to insert into the string
$r = " class=\"ext-link\"";
// $iR is length (+1) of $r to add to marker to account for string padding after insertion
$iR = strlen( $r ) + 1;
// while the position marker is before the end of the string,
while( $pN < strlen( $t ) ) {
// get the position of the first element of the needle array
// ( if the element does not appear, the result is FALSE )
$p[0] = strpos( $t, $n[0], $pN );
// if section beginning exists,
if( $p[0] ) {
// get the position of the remaining elements of the needle array
for( $i = 1; $i < count( $n ); $i++ ) {
$p[$i] = strpos( $t, $n[$i], $p[0] );
}
// move the marker to the section end and
$pN = $p[2] + 1;
// if trigger term exists within the section,
if( $p[1] && ( $p[1] < $p[2] ) ) {
// check for the exclusion terms –
for( $j = 3; $j < count( $n ); $j++ ) {
if( $p[$j] && ( $p[$j] < $p[2] ) ) {
// if ANY exist within the section, move the marker to the section end and restart the while loop
$pN = $p[2] + 1;
continue 2;
}
}
// set position for insertion at 2 past section beginning
$pR = $p[0] + 2;
// insert $r
$t = substr_replace( $t, $r, $pR, 0 );
// move the marker to the section end (padded to account for the insertion text length
$pN = $p[2] + $iR;
}
// if there is no section beginning string, break the loop
} else break;
}
return $t;
}

Since WP adds the title between '<a ' and 'href=', the above code ignores any posts with titles. We entitle all posts on our blog, so the replacement I have suggested works for us. Of course, if you only entitle _some_, neither solution works for everything.

My $0.02. I will post a more comprehensive solution if I ever figure out how to wrangle regular expressions to auto-detect and keep those from being updated.

Cheers
C