nwildlsearch regex case, does 'match' not 'search'?

Hi,

It seems that using ${lookup {..}nwildlsearch{myfile}{true}{false}} does
wat python would call a 'match' instead of a 'search' when using the '^'
character to indicate the keys are regexps?
Or rather, the '^' to indicate the key is a regexp, also seems to be
handled as the Beginning Of Line anchor in regexp lingo.

This isn't made clear in the docs, or did i miss it?

Given:

| $ cat /tmp/myfile
| ^My Subject
| 
| $ exim4 -be
| > ${lookup {THiS is MY SubJEcT}nwildlsearch{/tmp/myfile}{true}{false}}
| false

And:

| $ cat /tmp/myfile
| ^.*My Subject
| 
| $ exim4 -be
| > ${lookup {THiS is MY SubJEcT}nwildlsearch{/tmp/myfile}{true}{false}}
| true

Also, when using `exim4 -be`, it isn't immediately clear that a repeated
lookup will yield a cached result. When trying to debug lookups, this is
quite confusing until you realise you're looking at a cached answer.

I would try to improve this part of the documentation and provide PR/MR
for this but the repo seems sort of closed off for this. :-)

Can i help? Submit a diff to exim-dev?

Regards,
-Sndr.
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