Introduction to Parsing in Java

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time to strip whitespace since whitespace (apart from newlines) is allowed as part of a name, yet we don t want any leading or trailing whitespace For the color parser element we have speci ed that the rst character must be a # followed by exactly six hexadecimal digits (seven characters in all), or a sequence of alphanumeric characters with the rst character alphabetic We have chosen to handle empty nodes specially We de ne an empty node as a left bracket followed by a right bracket, and replace the brackets with the value EmptyBlock which earlier in the le is de ned as EmptyBlock = 0 This means that in the parser s results list we represent empty blocks with 0, and as noted earlier, we represent new rows by an integer row count (which will always be > 0)
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nodes = Forward() node_data = Optional(color + Suppress(":")) + Optional(name) node_datasetParseAction(add_block) node = left_bracket - node_data + nodes + right_bracket nodes << Group(ZeroOrMore(Optional(new_rows) + OneOrMore(node | empty_node)))
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We de ne nodes to be a Forward() parser element, since we need to use it before we specify what it matches We have also introduced a new parser element that isn t in the BNF, node_data, which matches the optional color and optional name We give this parser element a parse action that will create a new Block, so each time a node_data is encountered a Block will be added to the parser s results list The node parser element is de ned very naturally as a direct translation of the BNF Notice that both the node_data and nodes parser elements are optional (the former consisting of two optional elements, the latter quanti ed by zero or more), so empty nodes are correctly allowed Finally, we can de ne the nodes parser element Since it was originally created as a Forward() we must append parser elements to it using << Here we have set nodes to be zero or more of an optional new row and one or more nodes Notice that we put node before empty_node since PyParsing matches left to right we normally order parser elements that have common pre xes from longest to shortest matching We have also grouped the nodes parser element s results using Group() this ensures that each nodes is created as a list in its own right This means that a node that contains nodes will be represented by a Block for the node, and by a list for the contained nodes and which in turn may contain Blocks, or integers for empty nodes or new rows, and so on It is because of this recursive structure that we had to create nodes as a Forward(), and also why we must use the << operator (which in PyParsing is used to append), to add the Group() parser element and the elements it contains to the nodes element
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One important but subtle point to note is that we used the - operator rather than the + operator in the de nition of the node parser element We could just as easily have used +, since both + (ParserElement__add__()) and - (ParserElement__sub__()) do the same job they return a parser element that represents the concatenation of the two parser elements that are the operator s operands The reason we chose to use - rather than + is due to a subtle but important difference between them The - operator will stop parsing and raise a ParseSyntaxException as soon as an error is encountered, something that the + operator doesn t do If we had used + all errors would have a line number of 1 and a column of 1; but by using -, any errors have the correct line and column numbers In general, using + is the right approach, but if our tests show that we are getting incorrect error locations, then we can start to change +s into -s as we have done here and in this case only a single change was necessary
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def add_block(tokens): return BlockBlock(tokensname, tokenscolor if tokenscolor else "white")
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Whenever a node_data is parsed instead of the text being returned and added to the parser s results list, we create and return a Block We also always set the color to white unless a color is explicitly speci ed In the previous examples we parsed a le and an open le handle (an opened
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ioTextIOWrapper); here we will parse a string It makes no difference to PyParsing whether we give it a string or a le, so long as we use ParserElement parseFile() or ParserElementparseString() as appropriate In fact, PyParsing offers other parsing methods, including ParserElementscanString() which searches a string for matches, and ParserElementtransformString() which re-
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turns a copy of the string it is given, but with matched texts transformed into new texts by returning new text from parse actions
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stack = [Blockget_root_block()] try: results = nodesparseString(text, parseAll=True) assert len(results) == 1 items = resultsasList()[0] populate_children(items, stack) except (ParseException, ParseSyntaxException) as err: raise ValueError("Error {{0}}: syntax error, line " "{0}"format(errlineno)) return stack[0]
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This is the rst PyParsing parser where we have used the parser s results rather than created the data structures ourselves during the parsing process We expect the results to be returned as a list containing a single ParseResults
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