Infatti funziona benissimo.

Ecco il file test2133.txt
codice:
 clicca_qui 

fsdsfksd,fqsdf

sfkqsdfomqsfpqsdpfqsd  clicca_qui 

sfk,qsf,  fisfiosif  sfiipoqsfjkoqspof s iksjios f sfciqsfi sf clicca_qui 

xvxdlgfvdpgvfpdg  sfsfklsfl  clicca_qui 

 clicca_qui  djnsdfnhisqfd bnsdufosqi
Ecco il codice PHP.

codice:
<? 
$a = "<A href=\"www.sito:80/file.qualcosa\" target=\"dove\"> clicca_qui </A> pippo pluto caio <A href=\"http://forum.html.it/forum/showthread.php?s=&threadid=840768\" target=\"dove\"> clicca_qui </A>";
$pattern ='(]*>)([^<]*)()';
$b = eregi_replace($pattern,"\\1\\2:\\3\\4",$a);
print $b;
?>
<hr>
<?
ob_start();
readFile("test2133.txt");
$contents = stripslashes(nl2br(ob_get_contents ()));
ob_end_clean();
$pattern ='(]*>)([^<]*)()';

$contents = eregi_replace($pattern,"\\1\\2:\\3\\4",$contents);
print $contents;
 ?>