arturo is right, the problem is too complex to be handled just using regexp's (what about the base element?)

Nonetheless I'll give it a go, as it demonstrate one of my favourite techniques... I just grab the href attribute in the first regexp and then use a subroutine to analyze it and modify it appropriately. I think this should work for simple links:

#!/bin/perl -w
use strict;

my $server = "http://www.foo.com";
my $path   = "/absolute/path/";
while( <DATA>)
  { s{<\s*a\s*href=(['"])(.*?)\1\s*>([^<]*)</a>}
     { build_link( $2, $3) }seg;                             # hand ou
+t the updating to a subroutine
    print;
  }

sub build_link                                               # here we
+ can play with the href attribute
  { my( $href, $text)= @_;
    if( $href=~ m{^(http|telnet|gopher|file|wais|ftp)://})
      { return qq{<a href="$href">$text</a>}; }
    elsif( $href=~ m{^/})
      { return qq{<a href="$server$href">$text</a>}; }
    else
      { return qq{<a href="$server$path$href">$text</a>}; }
  }

__DATA__
<a href="/absolute/no/dns">absolute with no dns</a>
<a href="http://absolute.with/dns.html">http://absolute.with/dns.html<
+/a>
<a href="relative/without/dns.html">relative/without/dns.html</a>
<a href="relative2/without/dns.html">relative2 without dns.html</a>
<a href="relative/without/dns.html">relative/without/dns.html</a>
<a name="toto">toto</a>
[download]