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    <p>If you want to capture all the structure of the your input, a parser is the only way to go. One can code a parser manually top-down recursive, but there is little point in doing that, which is why parser generator tools exist; use them.</p> <p>Regarding the #if #then #else: if that's the <em>only</em> structure you want to capture, then you need only a pretty primitive grammar that also allows tokens containing arbitrary text to pick up the goo between the #if#then#else constructs as a blob of text.</p> <p>If you want to capture all code structure, and the conditionals are only allowed in certain places, then their existence can be simply integrated into whatever BNF you are using.</p> <p>If, as I suspect, these can occur <em>anywhere</em> ("ad hoc"? the #if follows C preprocessor style, and those conditionals can occur virtually anywhere in the input stream), then parsing the text <em>and</em> retaining the conditionals is presently at the bleeding edge of what state of the art parsing can do. This is the standard C-preprocessing disease, and there have been no good solutions to this. Standard parser generators pretty can't help in this case. (Hand coded parsers don't fare better here either; the same kind of solution has to be used in either case).</p> <p>One of the recent schemes (just reported as PhD research results in the last few months) to handle this is to fork the parse whenever a #if token is found to handle #if, and #else, and join when #endif is found; then you need a way to fuse to the generated subtrees typically as ambiguous subtrees marked with which arm of the conditional.</p> <p>If you want to get on with your life, I suggest you simply insist that these conditionals occur in well-defined places in your grammar, and put up with the occasional complaint from people that write unstructured preprocessor directives. ("You wrote crazy code? Sorry, my tool doesn't handle it").</p>
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    1. PORecommended strategy for parsing ad-hoc if/else syntax in Java?
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    1. PORecommended strategy for parsing ad-hoc if/else syntax in Java?
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    1. COBig thanks for the detailed answer! Indeed, the #if#else stuff occurs only inside the <command> tag, AFAIK (maybe wasn't too clear about it). As said, not too sure about if "ad-hoc" is the right term here (sorry, not a native english spaker :s) ... maybe "custom" is more appropriate ... hmm ...
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      1. USSamuel Lampa
    2. COLooking at your example more carefully, it appears what you are preprocessor style conditionals used to control Python code. That's exactly the same issue as using them to control C code. The question you have to answer is, "are they used in a structured way or not?". You can answer it by simply using a parser, which will insist on accepting #if#then#else only where you allow it.
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    3. COAh, ok ... If so, that might explain why I haven't found the code for parsing the #if#else syntax in the galaxy code itself, eventhough grepping through the code. Yea ... sounds like a good way to go. :)
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