Disminución de todos los encabezados en ASP clásico

You’re asking for pain trying to do this with regex (not so much the replace but the fact that it's an increment with a single regex pattern), if it's only a case of replacing the Headers, i'd use replace():

For i = 4 To 1 Step -1
    strHtml = replace(strHtml, "<h" & cstr(i), "<h" & cstr(i + 2), 1, -1, vbTextCompare)
    strHtml = replace(strHtml , "</h" & cstr(i), "</h" & cstr(i + 2), 1, -1, vbTextCompare)
Next

(HTML Spec is only valid for H1-H6 - not sure if you want to ignore H5 & H6)

If you want to stick with the regex option, i'd suggest the use of regex.replace()

I know in JavaScript you can pass the matched pattern into a function and use that function as the replacement, exactly what you would need here - but i've never seen this done in VBSCRIPT, example: Use RegExp para hacer coincidir un número entre paréntesis y luego increméntelo

Editar 1:

Found the reference to the matches collection & match object:

http://msdn.microsoft.com/en-us/library/ms974570.aspx#scripting05_topic3

So, you could read the match from the match.value property, but you'd still need to resort to a 2nd replace i think

Here's a more general solution, probably off-topic, though, as it is in Perl, not in VBScript. Note I documented it to counter the write-only effect that regular expressions tend to have.

C:\TEMP :: more /t4 hdradj.pl
use strict;
use warnings;

# Make a subroutine that will adjust HTML headers
# (like h1, H2) by doing string replacement.
# Will only work properly for the range from 1 to 9.
sub mk_header_adjuster {
    my( $adjustment, $lower, $upper ) = @_;
# Compile substitution expression to adjust headers like h1, H2.
# Left-hand side matches headers from the range specified.
# Uses word boundaries (\b) and a character range (square brackets).
# Captures matches for "h|H" in $1 and for the level in $2.
# Right-hand side uses an eval (e-switch) to compute substitution.
# Case is ignored (i-switch), replacement is global (g-switch).
# Wraps expression in subroutine to modify first argument.
    my $str = <<"EOSUB";
sub {
    \$_[0] =~ s/\\b(h)([$lower-$upper])\\b/\$1 . (\$2 + $adjustment)/ige
}
EOSUB
#   print $str, "\n"; # debug output
    my $sub = eval $str; # compile expression
    die $@ if $@; # abort in case of errors in eval compilation
    return $sub;
}

# ==== main ====
# Test the above subroutine is working properly.
my $test_input = <<'EOS';
<h1>eins</h1>
<p>...
<h2 bla="blub">eins blub</h2>
< H2 >zwei </ H2>
<h3 >drei </h3>
<h4>vier </h4>
<h5>fünf </h5>
EOS

# Compile a header adjuster moving headers 2 to 4 by 2 levels down.
my $adjuster = mk_header_adjuster 2, 2, 4;
my $number_of_replacements = $adjuster->( $test_input );
printf STDERR
"Replaced %u header tags in supplied input.\n", $number_of_replacements;
print $test_input;