Parser for xml DTD file

Question

I am quite new in implementing a parser and I am trying to pars a xml DTD file to generate a context free grammar for it. I tried pyparsing and yacc but still I could get any result. So I would appreciate if some one could provide me some tips or sample code to write such a parser. following is a sample DTD file:

<!DOCTYPE PcSpecs [
<!ELEMENT PCS (PC*)>
<!ELEMENT PC (MODEL, PRICE, PROCESSOR, RAM, DISK+)>
<!ELEMENT MODEL (\#PCDATA)>
<!ELEMENT PRICE (\#PCDATA)>
<!ELEMENT PROCESSOR (MANF, MODEL, SPEED)>
<!ELEMENT MANF (\#PCDATA)>
<!ELEMENT MODEL (\#PCDATA)>
<!ELEMENT SPEED (\#PCDATA)>
<!ELEMENT RAM (\#PCDATA)>
<!ELEMENT DISK (HARDDISK | CD | DVD)>
<!ELEMENT HARDDISK (MANF, MODEL, SIZE)>
<!ELEMENT SIZE (\#PCDATA)>
<!ELEMENT CD (SPEED)>
<!ELEMENT DVD (SPEED)>
]>

Thanks in advance.

Answer 1

Here's a start for you, it will parse the data into a ParseResults data structure, which you can then walk and create a parser for the defined doctype:

from pyparsing import *

LT,GT,EXCLAM,LBRACK,RBRACK,LPAR,RPAR = map(Suppress,"<>![]()")
DOCTYPE = Keyword("DOCTYPE").suppress()
ELEMENT = Keyword("ELEMENT").suppress()
ident = Word(alphas, alphanums+"_")
elementRef = Group(ident("name") + Optional(oneOf("* +")("rep")))
elementExpr = infixNotation(elementRef,
    [
    (',', 2, opAssoc.LEFT),
    ('|', 2, opAssoc.LEFT),
    ])
PCDATA = Literal(r"\#PCDATA")
elementDefn = Group(LT+EXCLAM + ELEMENT + ident("name") + 
                  LPAR + (elementExpr | PCDATA("PCDATA"))("contents") + RPAR + GT)
doctypeDefn = LT+EXCLAM + DOCTYPE + ident("name") + 
                    LBRACK + ZeroOrMore(elementDefn)("elements") + RBRACK + GT

I had started to just use a delimitedList for the list of elements in each ELEMENT definition, but then I noticed that ',' and '|' are actually operators, not just delimiters, and could even be mixed, as in "A,B,C|D,E". So I used pyparsing's infixNotation helper to allow these kinds of definitions.

With your input sample, I can parse and display the results with:

doctype = doctypeDefn.parseString(sample)
print doctype.dump()
for elem in doctype.elements:
    print elem.dump()

giving:

['PcSpecs', ['PCS', ['PC', '*']], ['PC', [['MODEL'], ...
- elements: [['PCS', ['PC', '*']], ['PC', [['MODEL'], ...
- name: PcSpecs
['PCS', ['PC', '*']]
- contents: ['PC', '*']
  - name: PC
  - rep: *
- name: PCS
['PC', [['MODEL'], ',', ['PRICE'], ',', ['PROCESSOR'], ',', ['RAM'], ',', ['DISK', '+']]]
- contents: [['MODEL'], ',', ['PRICE'], ',', ['PROCESSOR'], ',', ['RAM'], ',', ['DISK', '+']]
- name: PC
['MODEL', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: MODEL
['PRICE', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: PRICE
['PROCESSOR', [['MANF'], ',', ['MODEL'], ',', ['SPEED']]]
- contents: [['MANF'], ',', ['MODEL'], ',', ['SPEED']]
- name: PROCESSOR
['MANF', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: MANF
['MODEL', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: MODEL
['SPEED', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: SPEED
['RAM', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: RAM
['DISK', [['HARDDISK'], '|', ['CD'], '|', ['DVD']]]
- contents: [['HARDDISK'], '|', ['CD'], '|', ['DVD']]
- name: DISK
['HARDDISK', [['MANF'], ',', ['MODEL'], ',', ['SIZE']]]
- contents: [['MANF'], ',', ['MODEL'], ',', ['SIZE']]
- name: HARDDISK
['SIZE', '\\#PCDATA']
- PCDATA: \#PCDATA
- contents: \#PCDATA
- name: SIZE
['CD', ['SPEED']]
- contents: ['SPEED']
  - name: SPEED
- name: CD
['DVD', ['SPEED']]
- contents: ['SPEED']
  - name: SPEED
- name: DVD