[Phpwm] Weird mod-rewrite problem

[Mike Tipping]

I have site that takes several parameters on the url and loads a survey
accordingly. To make this easier I have used mod_rewrite so I have a url
that looks like:

http://www.frontrange-sam.com/dm/crc/ya6hft/

The last 6 figure code is a random code that links to a recipients data to
repopulate the form.

The modrewite rule for this is:

RewriteRule ^dm/([^/\.]+)/([^/\.]+)/
index.php?code=$2&regcode=edm&source=index&lang=$1 [L]

The issue I have is with 'h' in the code. If you put a h after the third
character it seems to mess up the mod rewrite rule.

So 

http://www.frontrange-sam.com/dm/crc/ya6xft/

And

http://www.frontrange-sam.com/dm/crc/hh3445/ work

But 

http://www.frontrange-sam.com/dm/crc/ya6hft/ don't

Any ideas what might be causing this?

Cheers

Mike