By Paul McGuire

Have to extract facts from a textual content dossier or an internet web page? Or do you need to make your program extra versatile with user-defined instructions or seek strings? Do average expressions and lex/yacc make your eyes blur and your mind hurt?

Pyparsing may be the resolution. Pyparsing is a pure-Python type library that makes it effortless to construct recursive-descent parsers fast. there isn't any have to handcraft your individual parsing kingdom computer. With pyparsing, you could quick create HTML web page scrapers, logfile facts extractors, or complicated info constitution or command processors. This brief minimize exhibits you the way!

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Extra resources for Getting Started with Pyparsing

Sample text

Binary operations in operatorPrecedence can return multiple operation/operands if multiple operations of the same precedence level occur together. For instance, a binary operation for addition will parse "a+b+c" as [ 'a', '+', 'b', '+', 'c' ], not Getting Started with Pyparsing 54 [[ 'a', '+', 'b' ], '+', 'c' ], so BinaryOperation needs to be able to recognize and process a chain of like operations, not just a simple a op b operation. The Python [0::2] slicing notation indicates that the operands will be taken from the list of tokens beginning with the 0th element, and then stepping by 2 until the end of the list.

Following our standard practice, we start by defining the BNF for an S-expression: alphaword ::= alphas+ integer ::= nums+ sexp ::= alphaword | integer | '(' sexp* ')' Getting Started with Pyparsing 36 The first two expressions are nothing new, and you can see how we would use pyparsing's Word class to define them: alphaword = Word(alphas) integer = Word(nums) But sexp is more difficult. Our dilemma is that the definition for sexp includes sexp itself, but to define sexp we need to refer to sexp!

In this version of the BNF, AND expressions will be parsed ahead of OR expressions, and NOT expressions will be parsed ahead of AND expressions. searchExpr ::= searchAnd [ OR searchAnd ]... searchAnd ::= searchTerm [ AND searchTerm ]... setParseAction( removeQuotes ) | \ Group( LPAREN + searchExpr + RPAREN ) ) searchAnd = Group( searchTerm + ZeroOrMore( and_ + searchTerm ) ) searchExpr << Group( searchAnd + ZeroOrMore( or_ + searchAnd ) ) Using this grammar, our parsing examples evaluate as: wood and blue or red [[['wood', 'and', 'blue'], 'or', ['red']]] wood and (blue or red) [[['wood', 'and', [[['blue'], 'or', ['red']]]]]] (steel or iron) and "lime green" [[[[[['steel'], 'or', ['iron']]], 'and', 'lime green']]] not steel or iron and "lime green" [[['not', 'steel'], 'or', ['iron', 'and', 'lime green']]] not(steel or iron) and "lime green" [[['not', [[['steel'], 'or', ['iron']]], 'and', 'lime green']]] These results can now be evaluated using a depth-first recursive method, and the AND, OR, and NOT expressions will be evaluated in the proper precedence.

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