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POPyparsing: a list of optional elements: weird issue with Optional, Each, and ordering of parser elements
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2012-06-15T12:47:05.760
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I'm trying to parse an XML-like file (with no associated DTD) with pyparsing. Part of each record looks has the following contents: <ul> <li>Something within <code><L></code> and <code><L/></code> tags,</li> <li>One or more things within <code><pc></code> and <code><pc/></code> tags,</li> <li>Optionally, something within <code><MW></code> and <code><MW/></code> tags,</li> <li>Optionally, a literal <code><mul/></code>, and optionally a literal <code><mat/></code></li> </ul> The ordering of these elements varies. So I wrote the following (I'm new to pyparsing; please point out if I'm doing something stupid): <pre><code>#!/usr/bin/env python from pyparsing import * def DumbTagParser(tag): tag_close = '</%s>' % tag return Group( Literal('<') + Literal(tag).setResultsName('tag') + Literal('>') + SkipTo(tag_close).setResultsName('contents') + Literal(tag_close) ).setResultsName(tag) record1 = Group(ZeroOrMore(DumbTagParser('pc'))).setResultsName('pcs') &\ DumbTagParser('L') & \ Optional(Literal('<mul/>')) & \ Optional(DumbTagParser('MW')) & \ Optional(Literal('<mat/>')) record2 = Group(ZeroOrMore(DumbTagParser('pc'))).setResultsName('pcs') &\ Optional(DumbTagParser('MW')) & \ Optional(Literal('<mul/>')) & \ DumbTagParser('L') def attempt(s): print 'Attempting:', s match = record1.parseString(s, parseAll = True) print 'Match: ', match print attempt('<L>1.1</L>') attempt('<pc>Page1,1</pc> <pc>Page1,2</pc> <MW>000001</MW> <L>1.1</L>') attempt('<mul/><MW>000003</MW><pc>1,1</pc><L>3.1</L>') attempt('<mul/> <MW>000003</MW> <pc>1,1</pc> <L>3.1</L> ') # Note end space </code></pre> Both parsers <code>record1</code> and <code>record2</code> fail, with different exceptions. With <code>record1</code>, it fails on the last string (which differs from the penultimate string only in spaces): <pre><code>pyparsing.ParseException: (at char 47), (line:1, col:48) </code></pre> and with <code>record2</code>, it fails on the penultimate string itself: <pre><code>pyparsing.ParseException: Missing one or more required elements (Group:({"<" "L" ">" SkipTo:("</L>") "</L>"})) (at char 0), (line:1, col:1) </code></pre> Now what is weird is that if I interchange lines 2 and 3 in the definition of <code>record2</code>, then it parses fine! <pre><code>record2 = Group(ZeroOrMore(DumbTagParser('pc'))).setResultsName('pcs') &\ Optional(Literal('<mul/>')) & \ Optional(DumbTagParser('MW')) & \ DumbTagParser('L') # parses my example strings fine </code></pre> (Yes I realise that <code>record2</code> doesn't contain any rule for <code><mat/></code>. I'm trying to get a minimal example that reflects this sensitivity to reordering.) I'm not sure if this is a bug in pyparsing or in my code, but my real question is how I should parse the kind of strings I want. 
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Pyparsing: a list of optional elements: weird issue with Optional, Each, and ordering of parser elements
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