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$catreg = "^(<[A-Za-z0-9]+>)*<H1>(<[A-Za-z0-9]+>)*([A-Za-z0-9]+)";

if ( m!$catreg! ) {

m!^(<[A-Za-z0-9]+>)*<H1>(<[A-Za-z0-9]+>)*([A-Za-z0-9]+)!

              # the lack of a $var =~ here indicates
              # that we are matching against $_
...
([A-Za-z0-9]+)    # matches sequential alphanumerics and
                  # captures all into $3
!                 # end of regex

$catreg = "^(<[A-Za-z0-9]+>)*<H1>(<[A-Za-z0-9]+>)*([A-Za-z0-9]+)";

...
} else {
    print "No Match\n";
}

$_='<h1>foo</h1>blah<H1>bar</H1>more blah';

while (m!<h1>(.*?)</h1>!ig) {
    print "Found $1\n";
}