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I want to write a python code generator, and it would be helpful if I had the regular expression that describes all valid python programs.

Does such a regular expression exist? What is it?

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    A regular expression? WHY? What are you trying to accomplish with that? It sounds like a truly awful idea, and most probably impossible. Commented Jul 3, 2012 at 6:08
  • As others have said, regular expressions are not sufficient to describes python syntax. However, the relatively short grammar that describes them can be seen in the source code here hg.python.org/cpython/file/d7bed032520b/Grammar/Grammar Commented Jul 3, 2012 at 8:10

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Such a regular expression cannot exist, because regular expressions are, by definition, not powerful enough to recognize Turing complete languages (such as python).

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My + 1. For example, regular expressions cannot be used to accept the nested pair structures -- like nested parentheses in expressions.
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If you generate a string with fragment of a Python code, and you want to check whether it is syntactically correct, try the built-in function compile. It returns the resulting code object or it raises the SyntaxError exception.

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You'll want to lint your code. There are a couple of tools to do that with Python; pylint, PyChecker and pyflakes should all do the trick.

To answer your actual question: no, this is not a job for regular expressions.

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