PyParsing and Headaches

Chris Lambacher chris at kateandchris.net
Wed Nov 22 20:50:30 CET 2006


On Wed, Nov 22, 2006 at 11:17:52AM -0800, Bytter wrote:
> Hi,
> 
> I'm trying to construct a parser, but I'm stuck with some basic
> stuff... For example, I want to match the following:
> 
> letter = "A"..."Z" | "a"..."z"
> literal = letter+
> include_bool := "+" | "-"
> term = [include_bool] literal
> 
> So I defined this as:
> 
> literal = Word(alphas)
> include_bool = Optional(oneOf("+ -"))
> term = include_bool + literal
+ here means that you allow a space.  You need to explicitly override this.
Try:

term = Combine(include_bool + literal)

> 
> The problem is that:
> 
> term.parseString("+a") -> (['+', 'a'], {}) # OK
> term.parseString("+ a") -> (['+', 'a'], {}) # KO. It shouldn't
> recognize any token since I didn't said the SPACE was allowed between
> include_bool and literal.
> 
> Can anyone give me an hand here?
> 
> Cheers!
> 
> Hugo Ferreira
> 
> BTW, the following is the complete grammar I'm trying to implement with
> pyparsing:
> 
> ## L ::= expr | expr L
> ## expr ::= term | binary_expr
> ## binary_expr ::= term " " binary_op " " term
> ## binary_op ::= "*" | "OR" | "AND"
> ## include_bool ::= "+" | "-"
> ## term ::= ([include_bool] [modifier ":"] (literal | range)) | ("~"
> literal)
> ## modifier ::= (letter | "_")+
> ## literal ::= word | quoted_words
> ## quoted_words ::= '"' word (" " word)* '"'
> ## word ::= (letter | digit | "_")+
> ## number ::= digit+
> ## range ::= number (".." | "...") number
> ## letter ::= "A"..."Z" | "a"..."z"
> ## digit ::= "0"..."9"
> 
> And this is where I got so far:
> 
> word = Word(nums + alphas + "_")
> binary_op = oneOf("* and or", caseless=True).setResultsName("operator")
> include_bool = oneOf("+ -")
> literal = (word | quotedString).setResultsName("literal")
> modifier = Word(alphas + "_")
> rng = Word(nums) + (Literal("..") | Literal("...")) + Word(nums)
> term = ((Optional(include_bool) + Optional(modifier + ":") + (literal |
> rng)) | ("~" + literal)).setResultsName("Term")
> binary_expr = (term + binary_op + term).setResultsName("binary")
> expr = (binary_expr | term).setResultsName("Expr")
> L = OneOrMore(expr)
> 
> 
> -- 
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> 
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