[Dovecot] sieve filtering, rewriting subjects

Is it possible to modify an emails subject line in a sieve script?

I've seen references to 'addtag' whilst googling, but it appears dovecots implementation doens't support it - are there any other methods?

As a side note, can anyone suggest a web based script editor that works? Simply a text area to write the script in would do

Thanks,

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Ian P. Christian ~ pookey.co.uk