Lexer in D

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For one of our courses this year we have a very cool topic: Lexer 
and Compiler.
In the past I have already tried some experiments in this 
direction with Romulus and Remus, but I've never measured my 
Lexing time.
That's why I wanted to start again to write a lexer (hitherto 
purely for the learning effect).
I looked at the code of some of the other lexers, that are also 
written in D, to get some inspiration from, but currently my 
Lexer is a little bit slow, it requires for example ~200 msecs 
for std.datetime.
So I wanted to ask whether there are nice people who help me to 
improve our lexer.

Code: https://github.com/Dgame/Puzzle/blob/master/lexer.d

Thanks in advance. :)