Write a programming language interpreter [duplicate]

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Possible Duplicate:

References Needed for Implementing an Interpreter in C/C++

How do I write a programming language interpreter such as how PHP works, im not talking about compilers though, are there any simple examples that can get me started writing a programming language interpreter?

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Have a look at boost.spirit - its a good place to start. (in particular - the writing parsers section of the docs)

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By no means a trivial amount of work if you are talking about a full featured programming language but of course doable with parsers and lexical analyzer such as flex