Parsing and splitting textfile

[Hugo Florentino]

Hello,

Can you point me to an efficient way to parse a text file and split it 
by certain expression (for example, `\n\nFrom\ .+ at .+$`), copying what 
has already been read to a separate file, and so on till the end of the 
file?

I am trying to implement a mailbox to maildir format conversion 
application in D, but I would like to avoid loading each mbox completely 
into memory.

Regards, Hugo