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HowToUsePyparsing.html
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HowToUsePyparsing.html
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<title>Using the pyparsing module</title>
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<div class="document" id="using-the-pyparsing-module">
<h1 class="title">Using the pyparsing module</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>Paul McGuire</td></tr>
<tr><th class="docinfo-name">Address:</th>
<td><pre class="address">
<a class="first last reference" href="mailto:ptmcg@users.sourceforge.net">ptmcg@users.sourceforge.net</a>
</pre>
</td></tr>
<tr><th class="docinfo-name">Revision:</th>
<td>1.5.0</td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>June, 2008</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright © 2003-2008 Paul McGuire.</td></tr>
</tbody>
</table>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">abstract:</th><td class="field-body">This document provides how-to instructions for the
pyparsing library, an easy-to-use Python module for constructing
and executing basic text parsers. The pyparsing module is useful
for evaluating user-definable
expressions, processing custom application language commands, or
extracting data from formatted reports.</td>
</tr>
</tbody>
</table>
<div class="contents topic">
<p class="topic-title first"><a id="contents" name="contents">Contents</a></p>
<ul class="auto-toc simple">
<li><a class="reference" href="#steps-to-follow" id="id1" name="id1">1 Steps to follow</a><ul class="auto-toc">
<li><a class="reference" href="#hello-world" id="id2" name="id2">1.1 Hello, World!</a></li>
<li><a class="reference" href="#new-features-in-1-5-0" id="id3" name="id3">1.2 New features in 1.5.0</a></li>
<li><a class="reference" href="#usage-notes" id="id4" name="id4">1.3 Usage notes</a></li>
</ul>
</li>
<li><a class="reference" href="#classes" id="id5" name="id5">2 Classes</a><ul class="auto-toc">
<li><a class="reference" href="#classes-in-the-pyparsing-module" id="id6" name="id6">2.1 Classes in the pyparsing module</a></li>
<li><a class="reference" href="#basic-parserelement-subclasses" id="id7" name="id7">2.2 Basic ParserElement subclasses</a></li>
<li><a class="reference" href="#expression-subclasses" id="id8" name="id8">2.3 Expression subclasses</a></li>
<li><a class="reference" href="#expression-operators" id="id9" name="id9">2.4 Expression operators</a></li>
<li><a class="reference" href="#positional-subclasses" id="id10" name="id10">2.5 Positional subclasses</a></li>
<li><a class="reference" href="#converter-subclasses" id="id11" name="id11">2.6 Converter subclasses</a></li>
<li><a class="reference" href="#special-subclasses" id="id12" name="id12">2.7 Special subclasses</a></li>
<li><a class="reference" href="#other-classes" id="id13" name="id13">2.8 Other classes</a></li>
<li><a class="reference" href="#exception-classes-and-troubleshooting" id="id14" name="id14">2.9 Exception classes and Troubleshooting</a></li>
</ul>
</li>
<li><a class="reference" href="#miscellaneous-attributes-and-methods" id="id15" name="id15">3 Miscellaneous attributes and methods</a><ul class="auto-toc">
<li><a class="reference" href="#helper-methods" id="id16" name="id16">3.1 Helper methods</a></li>
<li><a class="reference" href="#helper-parse-actions" id="id17" name="id17">3.2 Helper parse actions</a></li>
<li><a class="reference" href="#common-string-and-token-constants" id="id18" name="id18">3.3 Common string and token constants</a></li>
</ul>
</li>
</ul>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id1" id="steps-to-follow" name="steps-to-follow">1 Steps to follow</a></h1>
<p>To parse an incoming data string, the client code must follow these steps:</p>
<ol class="arabic simple">
<li>First define the tokens and patterns to be matched, and assign
this to a program variable. Optional results names or parsing
actions can also be defined at this time.</li>
<li>Call <tt class="docutils literal"><span class="pre">parseString()</span></tt> or <tt class="docutils literal"><span class="pre">scanString()</span></tt> on this variable, passing in
the string to
be parsed. During the matching process, whitespace between
tokens is skipped by default (although this can be changed).
When token matches occur, any defined parse action methods are
called.</li>
<li>Process the parsed results, returned as a list of strings.
Matching results may also be accessed as named attributes of
the returned results, if names are defined in the definition of
the token pattern, using <tt class="docutils literal"><span class="pre">setResultsName()</span></tt>.</li>
</ol>
<div class="section">
<h2><a class="toc-backref" href="#id2" id="hello-world" name="hello-world">1.1 Hello, World!</a></h2>
<p>The following complete Python program will parse the greeting "Hello, World!",
or any other greeting of the form "<salutation>, <addressee>!":</p>
<pre class="literal-block">
from pyparsing import Word, alphas
greet = Word( alphas ) + "," + Word( alphas ) + "!"
greeting = greet.parseString( "Hello, World!" )
print greeting
</pre>
<p>The parsed tokens are returned in the following form:</p>
<pre class="literal-block">
['Hello', ',', 'World', '!']
</pre>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id3" id="new-features-in-1-5-0" name="new-features-in-1-5-0">1.2 New features in 1.5.0</a></h2>
<p>Some of the significant features added in version 1.5.0 are:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">indentedBlock</span></tt> helper method to define grammars using block indentation for grouping (like Python)</li>
<li>new parameter <tt class="docutils literal"><span class="pre">parseAll</span></tt> in <tt class="docutils literal"><span class="pre">ParserElement.parseString</span></tt></li>
<li>operator '-' for combining ParserElements; similar to the '+' operator, but raises an immediate
<tt class="docutils literal"><span class="pre">ParseSyntaxException</span></tt> if an expression after the '-' operator fails to match; using '-' can
provide error messages that are more useful for application users to find syntax errors in their
input text</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id4" id="usage-notes" name="usage-notes">1.3 Usage notes</a></h2>
<ul>
<li><p class="first">The pyparsing module can be used to interpret simple command
strings or algebraic expressions, or can be used to extract data
from text reports with complicated format and structure ("screen
or report scraping"). However, it is possible that your defined
matching patterns may accept invalid inputs. Use pyparsing to
extract data from strings assumed to be well-formatted.</p>
</li>
<li><p class="first">To keep up the readability of your code, use <a class="reference" href="#operators">operators</a> such as <tt class="docutils literal"><span class="pre">+</span></tt>, <tt class="docutils literal"><span class="pre">|</span></tt>,
<tt class="docutils literal"><span class="pre">^</span></tt>, and <tt class="docutils literal"><span class="pre">~</span></tt> to combine expressions. You can also combine
string literals with ParseExpressions - they will be
automatically converted to Literal objects. For example:</p>
<pre class="literal-block">
integer = Word( nums ) # simple unsigned integer
variable = Word( alphas, max=1 ) # single letter variable, such as x, z, m, etc.
arithOp = Word( "+-*/", max=1 ) # arithmetic operators
equation = variable + "=" + integer + arithOp + integer # will match "x=2+2", etc.
</pre>
<p>In the definition of <tt class="docutils literal"><span class="pre">equation</span></tt>, the string <tt class="docutils literal"><span class="pre">"="</span></tt> will get added as
a <tt class="docutils literal"><span class="pre">Literal("=")</span></tt>, but in a more readable way.</p>
</li>
<li><p class="first">The pyparsing module's default behavior is to ignore whitespace. This is the
case for 99% of all parsers ever written. This allows you to write simple, clean,
grammars, such as the above <tt class="docutils literal"><span class="pre">equation</span></tt>, without having to clutter it up with
extraneous <tt class="docutils literal"><span class="pre">ws</span></tt> markers. The <tt class="docutils literal"><span class="pre">equation</span></tt> grammar will successfully parse all of the
following statements:</p>
<pre class="literal-block">
x=2+2
x = 2+2
a = 10 * 4
r= 1234/ 100000
</pre>
<p>Of course, it is quite simple to extend this example to support more elaborate expressions, with
nesting with parentheses, floating point numbers, scientific notation, and named constants
(such as <tt class="docutils literal"><span class="pre">e</span></tt> or <tt class="docutils literal"><span class="pre">pi</span></tt>). See <tt class="docutils literal"><span class="pre">fourFn.py</span></tt>, included in the examples directory.</p>
</li>
<li><p class="first">To modify pyparsing's default whitespace skipping, you can use one or
more of the following methods:</p>
<ul>
<li><p class="first">use the static method <tt class="docutils literal"><span class="pre">ParserElement.setDefaultWhitespaceChars</span></tt>
to override the normal set of whitespace chars (' tn'). For instance
when defining a grammar in which newlines are significant, you should
call <tt class="docutils literal"><span class="pre">ParserElement.setDefaultWhitespaceChars('</span> <span class="pre">\t')</span></tt> to remove
newline from the set of skippable whitespace characters. Calling
this method will affect all pyparsing expressions defined afterward.</p>
</li>
<li><p class="first">call <tt class="docutils literal"><span class="pre">leaveWhitespace()</span></tt> on individual expressions, to suppress the
skipping of whitespace before trying to match the expression</p>
</li>
<li><p class="first">use <tt class="docutils literal"><span class="pre">Combine</span></tt> to require that successive expressions must be
adjacent in the input string. For instance, this expression:</p>
<pre class="literal-block">
real = Word(nums) + '.' + Word(nums)
</pre>
<p>will match "3.14159", but will also match "3 . 12". It will also
return the matched results as ['3', '.', '14159']. By changing this
expression to:</p>
<pre class="literal-block">
real = Combine( Word(nums) + '.' + Word(nums) )
</pre>
<p>it will not match numbers with embedded spaces, and it will return a
single concatenated string '3.14159' as the parsed token.</p>
</li>
</ul>
</li>
<li><p class="first">Repetition of expressions can be indicated using the '*' operator. An
expression may be multiplied by an integer value (to indicate an exact
repetition count), or by a tuple containing
two integers, or None and an integer, representing min and max repetitions
(with None representing no min or no max, depending whether it is the first or
second tuple element). See the following examples, where n is used to
indicate an integer value:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">expr*3</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">expr</span> <span class="pre">+</span> <span class="pre">expr</span> <span class="pre">+</span> <span class="pre">expr</span></tt></li>
<li><tt class="docutils literal"><span class="pre">expr*(2,3)</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">expr</span> <span class="pre">+</span> <span class="pre">expr</span> <span class="pre">+</span> <span class="pre">Optional(expr)</span></tt></li>
<li><tt class="docutils literal"><span class="pre">expr*(n,None)</span></tt> or <tt class="docutils literal"><span class="pre">expr*(n,)</span></tt> is equivalent
to <tt class="docutils literal"><span class="pre">expr*n</span> <span class="pre">+</span> <span class="pre">ZeroOrMore(expr)</span></tt> (read as "at least n instances of expr")</li>
<li><tt class="docutils literal"><span class="pre">expr*(None,n)</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">expr*(0,n)</span></tt>
(read as "0 to n instances of expr")</li>
<li><tt class="docutils literal"><span class="pre">expr*(None,None)</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">ZeroOrMore(expr)</span></tt></li>
<li><tt class="docutils literal"><span class="pre">expr*(1,None)</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">OneOrMore(expr)</span></tt></li>
</ul>
<p>Note that <tt class="docutils literal"><span class="pre">expr*(None,n)</span></tt> does not raise an exception if
more than n exprs exist in the input stream; that is,
<tt class="docutils literal"><span class="pre">expr*(None,n)</span></tt> does not enforce a maximum number of expr
occurrences. If this behavior is desired, then write
<tt class="docutils literal"><span class="pre">expr*(None,n)</span> <span class="pre">+</span> <span class="pre">~expr</span></tt>.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">MatchFirst</span></tt> expressions are matched left-to-right, and the first
match found will skip all later expressions within, so be sure
to define less-specific patterns after more-specific patterns.
If you are not sure which expressions are most specific, use Or
expressions (defined using the <tt class="docutils literal"><span class="pre">^</span></tt> operator) - they will always
match the longest expression, although they are more
compute-intensive.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">Or</span></tt> expressions will evaluate all of the specified subexpressions
to determine which is the "best" match, that is, which matches
the longest string in the input data. In case of a tie, the
left-most expression in the <tt class="docutils literal"><span class="pre">Or</span></tt> list will win.</p>
</li>
<li><p class="first">If parsing the contents of an entire file, pass it to the
<tt class="docutils literal"><span class="pre">parseFile</span></tt> method using:</p>
<pre class="literal-block">
expr.parseFile( sourceFile )
</pre>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">ParseExceptions</span></tt> will report the location where an expected token
or expression failed to match. For example, if we tried to use our
"Hello, World!" parser to parse "Hello World!" (leaving out the separating
comma), we would get an exception, with the message:</p>
<pre class="literal-block">
pyparsing.ParseException: Expected "," (6), (1,7)
</pre>
<p>In the case of complex
expressions, the reported location may not be exactly where you
would expect. See more information under <a class="reference" href="#parseexception">ParseException</a> .</p>
</li>
<li><p class="first">Use the <tt class="docutils literal"><span class="pre">Group</span></tt> class to enclose logical groups of tokens within a
sublist. This will help organize your results into more
hierarchical form (the default behavior is to return matching
tokens as a flat list of matching input strings).</p>
</li>
<li><p class="first">Punctuation may be significant for matching, but is rarely of
much interest in the parsed results. Use the <tt class="docutils literal"><span class="pre">suppress()</span></tt> method
to keep these tokens from cluttering up your returned lists of
tokens. For example, <tt class="docutils literal"><span class="pre">delimitedList()</span></tt> matches a succession of
one or more expressions, separated by delimiters (commas by
default), but only returns a list of the actual expressions -
the delimiters are used for parsing, but are suppressed from the
returned output.</p>
</li>
<li><p class="first">Parse actions can be used to convert values from strings to
other data types (ints, floats, booleans, etc.).</p>
</li>
<li><p class="first">Results names are recommended for retrieving tokens from complex
expressions. It is much easier to access a token using its field
name than using a positional index, especially if the expression
contains optional elements. You can also shortcut
the <tt class="docutils literal"><span class="pre">setResultsName</span></tt> call:</p>
<pre class="literal-block">
stats = "AVE:" + realNum.setResultsName("average") + \
"MIN:" + realNum.setResultsName("min") + \
"MAX:" + realNum.setResultsName("max")
</pre>
<p>can now be written as this:</p>
<pre class="literal-block">
stats = "AVE:" + realNum("average") + \
"MIN:" + realNum("min") + \
"MAX:" + realNum("max")
</pre>
</li>
<li><p class="first">Be careful when defining parse actions that modify global variables or
data structures (as in <tt class="docutils literal"><span class="pre">fourFn.py</span></tt>), especially for low level tokens
or expressions that may occur within an <tt class="docutils literal"><span class="pre">And</span></tt> expression; an early element
of an <tt class="docutils literal"><span class="pre">And</span></tt> may match, but the overall expression may fail.</p>
</li>
<li><p class="first">Performance of pyparsing may be slow for complex grammars and/or large
input strings. The <a class="reference" href="http://psyco.sourceforge.net/">psyco</a> package can be used to improve the speed of the
pyparsing module with no changes to grammar or program logic - observed
improvments have been in the 20-50% range.</p>
</li>
</ul>
</div>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id5" id="classes" name="classes">2 Classes</a></h1>
<div class="section">
<h2><a class="toc-backref" href="#id6" id="classes-in-the-pyparsing-module" name="classes-in-the-pyparsing-module">2.1 Classes in the pyparsing module</a></h2>
<p><tt class="docutils literal"><span class="pre">ParserElement</span></tt> - abstract base class for all pyparsing classes;
methods for code to use are:</p>
<ul>
<li><p class="first"><tt class="docutils literal"><span class="pre">parseString(</span> <span class="pre">sourceString,</span> <span class="pre">parseAll=False</span> <span class="pre">)</span></tt> - only called once, on the overall
matching pattern; returns a <a class="reference" href="#parseresults">ParseResults</a> object that makes the
matched tokens available as a list, and optionally as a dictionary,
or as an object with named attributes; if parseAll is set to True, then
parseString will raise a ParseException if the grammar does not process
the complete input string.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">parseFile(</span> <span class="pre">sourceFile</span> <span class="pre">)</span></tt> - a convenience function, that accepts an
input file object or filename. The file contents are passed as a
string to <tt class="docutils literal"><span class="pre">parseString()</span></tt>.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">scanString(</span> <span class="pre">sourceString</span> <span class="pre">)</span></tt> - generator function, used to find and
extract matching text in the given source string; for each matched text,
returns a tuple of:</p>
<ul class="simple">
<li>matched tokens (packaged as a <a class="reference" href="#parseresults">ParseResults</a> object)</li>
<li>start location of the matched text in the given source string</li>
<li>end location in the given source string</li>
</ul>
<p><tt class="docutils literal"><span class="pre">scanString</span></tt> allows you to scan through the input source string for
random matches, instead of exhaustively defining the grammar for the entire
source text (as would be required with <tt class="docutils literal"><span class="pre">parseString</span></tt>).</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">transformString(</span> <span class="pre">sourceString</span> <span class="pre">)</span></tt> - convenience wrapper function for
<tt class="docutils literal"><span class="pre">scanString</span></tt>, to process the input source string, and replace matching
text with the tokens returned from parse actions defined in the grammar
(see <a class="reference" href="#setparseaction">setParseAction</a>).</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">searchString(</span> <span class="pre">sourceString</span> <span class="pre">)</span></tt> - another convenience wrapper function for
<tt class="docutils literal"><span class="pre">scanString</span></tt>, returns a list of the matching tokens returned from each
call to <tt class="docutils literal"><span class="pre">scanString</span></tt>.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">setName(</span> <span class="pre">name</span> <span class="pre">)</span></tt> - associate a short descriptive name for this
element, useful in displaying exceptions and trace information</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">setResultsName(</span> <span class="pre">string,</span> <span class="pre">listAllMatches=False</span> <span class="pre">)</span></tt> - name to be given
to tokens matching
the element; if multiple tokens within
a repetition group (such as <tt class="docutils literal"><span class="pre">ZeroOrMore</span></tt> or <tt class="docutils literal"><span class="pre">delimitedList</span></tt>) the
default is to return only the last matching token - if listAllMatches
is set to True, then a list of matching tokens is returned. Note:
<tt class="docutils literal"><span class="pre">setResultsName</span></tt> returns a <em>copy</em> of the element so that a single
basic element can be referenced multiple times and given
different names within a complex grammar.</p>
</li>
</ul>
<ul id="setparseaction">
<li><p class="first"><tt class="docutils literal"><span class="pre">setParseAction(</span> <span class="pre">*fn</span> <span class="pre">)</span></tt> - specify one or more functions to call after successful
matching of the element; each function is defined as <tt class="docutils literal"><span class="pre">fn(</span> <span class="pre">s,</span>
<span class="pre">loc,</span> <span class="pre">toks</span> <span class="pre">)</span></tt>, where:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">s</span></tt> is the original parse string</li>
<li><tt class="docutils literal"><span class="pre">loc</span></tt> is the location in the string where matching started</li>
<li><tt class="docutils literal"><span class="pre">toks</span></tt> is the list of the matched tokens, packaged as a <a class="reference" href="#parseresults">ParseResults</a> object</li>
</ul>
<p>Multiple functions can be attached to a ParserElement by specifying multiple
arguments to setParseAction, or by calling setParseAction multiple times.</p>
<p>Each parse action function can return a modified <tt class="docutils literal"><span class="pre">toks</span></tt> list, to perform conversion, or
string modifications. For brevity, <tt class="docutils literal"><span class="pre">fn</span></tt> may also be a
lambda - here is an example of using a parse action to convert matched
integer tokens from strings to integers:</p>
<pre class="literal-block">
intNumber = Word(nums).setParseAction( lambda s,l,t: [ int(t[0]) ] )
</pre>
<p>If <tt class="docutils literal"><span class="pre">fn</span></tt> does not modify the <tt class="docutils literal"><span class="pre">toks</span></tt> list, it does not need to return
anything at all.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">setBreak(</span> <span class="pre">breakFlag=True</span> <span class="pre">)</span></tt> - if breakFlag is True, calls pdb.set_break()
as this expression is about to be parsed</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">copy()</span></tt> - returns a copy of a ParserElement; can be used to use the same
parse expression in different places in a grammar, with different parse actions
attached to each</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">leaveWhitespace()</span></tt> - change default behavior of skipping
whitespace before starting matching (mostly used internally to the
pyparsing module, rarely used by client code)</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">setWhitespaceChars(</span> <span class="pre">chars</span> <span class="pre">)</span></tt> - define the set of chars to be ignored
as whitespace before trying to match a specific ParserElement, in place of the
default set of whitespace (space, tab, newline, and return)</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">setDefaultWhitespaceChars(</span> <span class="pre">chars</span> <span class="pre">)</span></tt> - class-level method to override
the default set of whitespace chars for all subsequently created ParserElements
(including copies); useful when defining grammars that treat one or more of the
default whitespace characters as significant (such as a line-sensitive grammar, to
omit newline from the list of ignorable whitespace)</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">suppress()</span></tt> - convenience function to suppress the output of the
given element, instead of wrapping it with a Suppress object.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">ignore(</span> <span class="pre">expr</span> <span class="pre">)</span></tt> - function to specify parse expression to be
ignored while matching defined patterns; can be called
repeatedly to specify multiple expressions; useful to specify
patterns of comment syntax, for example</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">setDebug(</span> <span class="pre">dbgFlag=True</span> <span class="pre">)</span></tt> - function to enable/disable tracing output
when trying to match this element</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">validate()</span></tt> - function to verify that the defined grammar does not
contain infinitely recursive constructs</p>
</li>
</ul>
<ul class="simple" id="parsewithtabs">
<li><tt class="docutils literal"><span class="pre">parseWithTabs()</span></tt> - function to override default behavior of converting
tabs to spaces before parsing the input string; rarely used, except when
specifying whitespace-significant grammars using the <a class="reference" href="#white">White</a> class.</li>
<li><tt class="docutils literal"><span class="pre">enablePackrat()</span></tt> - a class-level static method to enable a memoizing
performance enhancement, known as "packrat parsing". packrat parsing is
disabled by default, since it may conflict with some user programs that use
parse actions. To activate the packrat feature, your
program must call the class method ParserElement.enablePackrat(). If
your program uses psyco to "compile as you go", you must call
enablePackrat before calling psyco.full(). If you do not do this,
Python will crash. For best results, call enablePackrat() immediately
after importing pyparsing.</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id7" id="basic-parserelement-subclasses" name="basic-parserelement-subclasses">2.2 Basic ParserElement subclasses</a></h2>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">Literal</span></tt> - construct with a string to be matched exactly</li>
<li><tt class="docutils literal"><span class="pre">CaselessLiteral</span></tt> - construct with a string to be matched, but
without case checking; results are always returned as the
defining literal, NOT as they are found in the input string</li>
<li><tt class="docutils literal"><span class="pre">Keyword</span></tt> - similar to Literal, but must be immediately followed by
whitespace, punctuation, or other non-keyword characters; prevents
accidental matching of a non-keyword that happens to begin with a
defined keyword</li>
<li><tt class="docutils literal"><span class="pre">CaselessKeyword</span></tt> - similar to Keyword, but with caseless matching
behavior</li>
</ul>
<ul id="word">
<li><p class="first"><tt class="docutils literal"><span class="pre">Word</span></tt> - one or more contiguous characters; construct with a
string containing the set of allowed initial characters, and an
optional second string of allowed body characters; for instance,
a common Word construct is to match a code identifier - in C, a
valid identifier must start with an alphabetic character or an
underscore ('_'), followed by a body that can also include numeric
digits. That is, <tt class="docutils literal"><span class="pre">a</span></tt>, <tt class="docutils literal"><span class="pre">i</span></tt>, <tt class="docutils literal"><span class="pre">MAX_LENGTH</span></tt>, <tt class="docutils literal"><span class="pre">_a1</span></tt>, <tt class="docutils literal"><span class="pre">b_109_</span></tt>, and
<tt class="docutils literal"><span class="pre">plan9FromOuterSpace</span></tt>
are all valid identifiers; <tt class="docutils literal"><span class="pre">9b7z</span></tt>, <tt class="docutils literal"><span class="pre">$a</span></tt>, <tt class="docutils literal"><span class="pre">.section</span></tt>, and <tt class="docutils literal"><span class="pre">0debug</span></tt>
are not. To
define an identifier using a Word, use either of the following:</p>
<pre class="literal-block">
- Word( alphas+"_", alphanums+"_" )
- Word( srange("[a-zA-Z_]"), srange("[a-zA-Z0-9_]") )
</pre>
<p>If only one
string given, it specifies that the same character set defined
for the initial character is used for the word body; for instance, to
define an identifier that can only be composed of capital letters and
underscores, use:</p>
<pre class="literal-block">
- Word( "ABCDEFGHIJKLMNOPQRSTUVWXYZ_" )
- Word( srange("[A-Z_]") )
</pre>
<p>A Word may
also be constructed with any of the following optional parameters:</p>
<ul class="simple">
<li>min - indicating a minimum length of matching characters</li>
<li>max - indicating a maximum length of matching characters</li>
<li>exact - indicating an exact length of matching characters</li>
</ul>
<p>If exact is specified, it will override any values for min or max.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">CharsNotIn</span></tt> - similar to <a class="reference" href="#word">Word</a>, but matches characters not
in the given constructor string (accepts only one string for both
initial and body characters); also supports min, max, and exact
optional parameters.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">Regex</span></tt> - a powerful construct, that accepts a regular expression
to be matched at the current parse position; accepts an optional
flags parameter, corresponding to the flags parameter in the re.compile
method; if the expression includes named sub-fields, they will be
represented in the returned <a class="reference" href="#parseresults">ParseResults</a></p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">QuotedString</span></tt> - supports the definition of custom quoted string
formats, in addition to pyparsing's built-in dblQuotedString and
sglQuotedString. QuotedString allows you to specify the following
parameters:</p>
<ul class="simple">
<li>quoteChar - string of one or more characters defining the quote delimiting string</li>
<li>escChar - character to escape quotes, typically backslash (default=None)</li>
<li>escQuote - special quote sequence to escape an embedded quote string (such as SQL's "" to escape an embedded ") (default=None)</li>
<li>multiline - boolean indicating whether quotes can span multiple lines (default=False)</li>
<li>unquoteResults - boolean indicating whether the matched text should be unquoted (default=True)</li>
<li>endQuoteChar - string of one or more characters defining the end of the quote delimited string (default=None => same as quoteChar)</li>
</ul>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">SkipTo</span></tt> - skips ahead in the input string, accepting any
characters up to the specified pattern; may be constructed with
the following optional parameters:</p>
<ul class="simple">
<li>include - if set to true, also consumes the match expression
(default is false)</li>
<li>ignore - allows the user to specify patterns to not be matched,
to prevent false matches</li>
</ul>
</li>
</ul>
<ul class="simple" id="white">
<li><tt class="docutils literal"><span class="pre">White</span></tt> - also similar to <a class="reference" href="#word">Word</a>, but matches whitespace
characters. Not usually needed, as whitespace is implicitly
ignored by pyparsing. However, some grammars are whitespace-sensitive,
such as those that use leading tabs or spaces to indicating grouping
or hierarchy. (If matching on tab characters, be sure to call
<a class="reference" href="#parsewithtabs">parseWithTabs</a> on the top-level parse element.)</li>
<li><tt class="docutils literal"><span class="pre">Empty</span></tt> - a null expression, requiring no characters - will always
match; useful for debugging and for specialized grammars</li>
<li><tt class="docutils literal"><span class="pre">NoMatch</span></tt> - opposite of Empty, will never match; useful for debugging
and for specialized grammars</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id8" id="expression-subclasses" name="expression-subclasses">2.3 Expression subclasses</a></h2>
<ul>
<li><p class="first"><tt class="docutils literal"><span class="pre">And</span></tt> - construct with a list of ParserElements, all of which must
match for And to match; can also be created using the '+'
operator; multiple expressions can be Anded together using the '*'
operator as in:</p>
<pre class="literal-block">
ipAddress = Word(nums) + ('.'+Word(nums))*3
</pre>
<p>A tuple can be used as the multiplier, indicating a min/max:</p>
<pre class="literal-block">
usPhoneNumber = Word(nums) + ('-'+Word(nums))*(1,2)
</pre>
<p>A special form of <tt class="docutils literal"><span class="pre">And</span></tt> is created if the '-' operator is used
instead of the '+' operator. In the ipAddress example above, if
no trailing '.' and Word(nums) are found after matching the initial
Word(nums), then pyparsing will back up in the grammar and try other
alternatives to ipAddress. However, if ipAddress is defined as:</p>
<pre class="literal-block">
strictIpAddress = Word(nums) - ('.'+Word(nums))*3
</pre>
<p>then no backing up is done. If the first Word(nums) of strictIpAddress
is matched, then any mismatch after that will raise a ParseSyntaxException,
which will halt the parsing process immediately. By careful use of the
'-' operator, grammars can provide meaningful error messages close to
the location where the incoming text does not match the specified
grammar.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">Or</span></tt> - construct with a list of ParserElements, any of which must
match for Or to match; if more than one expression matches, the
expression that makes the longest match will be used; can also
be created using the '^' operator</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">MatchFirst</span></tt> - construct with a list of ParserElements, any of
which must match for MatchFirst to match; matching is done
left-to-right, taking the first expression that matches; can
also be created using the '|' operator</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">Each</span></tt> - similar to And, in that all of the provided expressions
must match; however, Each permits matching to be done in any order;
can also be created using the '&' operator</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">Optional</span></tt> - construct with a ParserElement, but this element is
not required to match; can be constructed with an optional <tt class="docutils literal"><span class="pre">default</span></tt> argument,
containing a default string or object to be supplied if the given optional
parse element is not found in the input string; parse action will only
be called if a match is found, or if a default is specified</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">ZeroOrMore</span></tt> - similar to Optional, but can be repeated</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">OneOrMore</span></tt> - similar to ZeroOrMore, but at least one match must
be present</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">FollowedBy</span></tt> - a lookahead expression, requires matching of the given
expressions, but does not advance the parsing position within the input string</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">NotAny</span></tt> - a negative lookahead expression, prevents matching of named
expressions, does not advance the parsing position within the input string;
can also be created using the unary '~' operator</p>
</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id9" id="expression-operators" name="expression-operators"><span id="operators"></span>2.4 Expression operators</a></h2>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">+</span></tt> - consecutive sequence</li>
<li><tt class="docutils literal"><span class="pre">|</span></tt> - match first alternative</li>
<li><tt class="docutils literal"><span class="pre">^</span></tt> - match longest alternative</li>
<li><tt class="docutils literal"><span class="pre">&</span></tt> - match each alternative, in any order</li>
<li><tt class="docutils literal"><span class="pre">-</span></tt> - like <tt class="docutils literal"><span class="pre">+</span></tt> but with no backup and retry of alternatives</li>
<li><tt class="docutils literal"><span class="pre">*</span></tt> - repetition of expression</li>
<li><tt class="docutils literal"><span class="pre">==</span></tt> - matching expression to string; returns True if the string matches the given expression</li>
<li><tt class="docutils literal"><span class="pre"><<</span></tt> - expression definition for Forward expressions</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id10" id="positional-subclasses" name="positional-subclasses">2.5 Positional subclasses</a></h2>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">StringStart</span></tt> - matches beginning of the text</li>
<li><tt class="docutils literal"><span class="pre">StringEnd</span></tt> - matches the end of the text</li>
<li><tt class="docutils literal"><span class="pre">LineStart</span></tt> - matches beginning of a line (lines delimited by <tt class="docutils literal"><span class="pre">\n</span></tt> characters)</li>
<li><tt class="docutils literal"><span class="pre">LineEnd</span></tt> - matches the end of a line</li>
<li><tt class="docutils literal"><span class="pre">WordStart</span></tt> - matches a leading word boundary</li>
<li><tt class="docutils literal"><span class="pre">WordEnd</span></tt> - matches a trailing word boundary</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id11" id="converter-subclasses" name="converter-subclasses">2.6 Converter subclasses</a></h2>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">Upcase</span></tt> - converts matched tokens to uppercase (deprecated -
use <tt class="docutils literal"><span class="pre">upcaseTokens</span></tt> parse action instead)</li>
<li><tt class="docutils literal"><span class="pre">Combine</span></tt> - joins all matched tokens into a single string, using
specified joinString (default <tt class="docutils literal"><span class="pre">joinString=""</span></tt>); expects
all matching tokens to be adjacent, with no intervening
whitespace (can be overridden by specifying <tt class="docutils literal"><span class="pre">adjacent=False</span></tt> in constructor)</li>
<li><tt class="docutils literal"><span class="pre">Suppress</span></tt> - clears matched tokens; useful to keep returned
results from being cluttered with required but uninteresting
tokens (such as list delimiters)</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id12" id="special-subclasses" name="special-subclasses">2.7 Special subclasses</a></h2>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">Group</span></tt> - causes the matched tokens to be enclosed in a list;
useful in repeated elements like <tt class="docutils literal"><span class="pre">ZeroOrMore</span></tt> and <tt class="docutils literal"><span class="pre">OneOrMore</span></tt> to
break up matched tokens into groups for each repeated pattern</li>
<li><tt class="docutils literal"><span class="pre">Dict</span></tt> - like <tt class="docutils literal"><span class="pre">Group</span></tt>, but also constructs a dictionary, using the
[0]'th elements of all enclosed token lists as the keys, and
each token list as the value</li>
<li><tt class="docutils literal"><span class="pre">SkipTo</span></tt> - catch-all matching expression that accepts all characters
up until the given pattern is found to match; useful for specifying
incomplete grammars</li>
<li><tt class="docutils literal"><span class="pre">Forward</span></tt> - placeholder token used to define recursive token
patterns; when defining the actual expression later in the
program, insert it into the <tt class="docutils literal"><span class="pre">Forward</span></tt> object using the <tt class="docutils literal"><span class="pre"><<</span></tt>
operator (see <tt class="docutils literal"><span class="pre">fourFn.py</span></tt> for an example).</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id13" id="other-classes" name="other-classes">2.8 Other classes</a></h2>
<ul id="parseresults">
<li><p class="first"><tt class="docutils literal"><span class="pre">ParseResults</span></tt> - class used to contain and manage the lists of tokens
created from parsing the input using the user-defined parse
expression. ParseResults can be accessed in a number of ways:</p>
<ul class="simple">
<li>as a list<ul>
<li>total list of elements can be found using len()</li>
<li>individual elements can be found using [0], [1], [-1], etc.</li>
<li>elements can be deleted using <tt class="docutils literal"><span class="pre">del</span></tt></li>
<li>the -1th element can be extracted and removed in a single operation
using <tt class="docutils literal"><span class="pre">pop()</span></tt>, or any element can be extracted and removed
using <tt class="docutils literal"><span class="pre">pop(n)</span></tt></li>
</ul>
</li>
<li>as a dictionary<ul>
<li>if <tt class="docutils literal"><span class="pre">setResultsName()</span></tt> is used to name elements within the
overall parse expression, then these fields can be referenced
as dictionary elements or as attributes</li>
<li>the Dict class generates dictionary entries using the data of the
input text - in addition to ParseResults listed as <tt class="docutils literal"><span class="pre">[</span> <span class="pre">[</span> <span class="pre">a1,</span> <span class="pre">b1,</span> <span class="pre">c1,</span> <span class="pre">...],</span> <span class="pre">[</span> <span class="pre">a2,</span> <span class="pre">b2,</span> <span class="pre">c2,</span> <span class="pre">...]</span> <span class="pre">]</span></tt>
it also acts as a dictionary with entries defined as <tt class="docutils literal"><span class="pre">{</span> <span class="pre">a1</span> <span class="pre">:</span> <span class="pre">[</span> <span class="pre">b1,</span> <span class="pre">c1,</span> <span class="pre">...</span> <span class="pre">]</span> <span class="pre">},</span> <span class="pre">{</span> <span class="pre">a2</span> <span class="pre">:</span> <span class="pre">[</span> <span class="pre">b2,</span> <span class="pre">c2,</span> <span class="pre">...</span> <span class="pre">]</span> <span class="pre">}</span></tt>;
this is especially useful when processing tabular data where the first column contains a key
value for that line of data</li>
<li>list elements that are deleted using <tt class="docutils literal"><span class="pre">del</span></tt> will still be accessible by their
dictionary keys</li>
<li>supports <tt class="docutils literal"><span class="pre">get()</span></tt>, <tt class="docutils literal"><span class="pre">items()</span></tt> and <tt class="docutils literal"><span class="pre">keys()</span></tt> methods, similar to a dictionary</li>
<li>a keyed item can be extracted and removed using <tt class="docutils literal"><span class="pre">pop(key)</span></tt>. Here
key must be non-numeric (such as a string), in order to use dict
extraction instead of list extraction.</li>
<li>new named elements can be added (in a parse action, for instance), using the same
syntax as adding an item to a dict (<tt class="docutils literal"><span class="pre">parseResults["X"]="new</span> <span class="pre">item"</span></tt>); named elements can be removed using <tt class="docutils literal"><span class="pre">del</span> <span class="pre">parseResults["X"]</span></tt></li>
</ul>
</li>
<li>as a nested list<ul>
<li>results returned from the Group class are encapsulated within their
own list structure, so that the tokens can be handled as a hierarchical
tree</li>
</ul>
</li>
</ul>
<p>ParseResults can also be converted to an ordinary list of strings
by calling <tt class="docutils literal"><span class="pre">asList()</span></tt>. Note that this will strip the results of any
field names that have been defined for any embedded parse elements.
(The <tt class="docutils literal"><span class="pre">pprint</span></tt> module is especially good at printing out the nested contents
given by <tt class="docutils literal"><span class="pre">asList()</span></tt>.)</p>
<p>Finally, ParseResults can be converted to an XML string by calling <tt class="docutils literal"><span class="pre">asXML()</span></tt>. Where
possible, results will be tagged using the results names defined for the respective
ParseExpressions. <tt class="docutils literal"><span class="pre">asXML()</span></tt> takes two optional arguments:</p>
<ul class="simple">
<li>doctagname - for ParseResults that do not have a defined name, this argument
will wrap the resulting XML in a set of opening and closing tags <tt class="docutils literal"><span class="pre"><doctagname></span></tt>
and <tt class="docutils literal"><span class="pre"></doctagname></span></tt>.</li>
<li>namedItemsOnly (default=False) - flag to indicate if the generated XML should
skip items that do not have defined names. If a nested group item is named, then all
embedded items will be included, whether they have names or not.</li>
</ul>
</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id14" id="exception-classes-and-troubleshooting" name="exception-classes-and-troubleshooting">2.9 Exception classes and Troubleshooting</a></h2>
<ul id="parseexception">
<li><p class="first"><tt class="docutils literal"><span class="pre">ParseException</span></tt> - exception returned when a grammar parse fails;
ParseExceptions have attributes loc, msg, line, lineno, and column; to view the
text line and location where the reported ParseException occurs, use:</p>
<pre class="literal-block">
except ParseException, err:
print err.line
print " "*(err.column-1) + "^"
print err
</pre>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">RecursiveGrammarException</span></tt> - exception returned by <tt class="docutils literal"><span class="pre">validate()</span></tt> if
the grammar contains a recursive infinite loop, such as:</p>
<pre class="literal-block">
badGrammar = Forward()
goodToken = Literal("A")
badGrammar << Optional(goodToken) + badGrammar
</pre>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">ParseFatalException</span></tt> - exception that parse actions can raise to stop parsing
immediately. Should be used when a semantic error is found in the input text, such
as a mismatched XML tag.</p>
</li>
<li><p class="first"><tt class="docutils literal"><span class="pre">ParseSyntaxException</span></tt> - subclass of <tt class="docutils literal"><span class="pre">ParseFatalException</span></tt> raised when a
syntax error is found, based on the use of the '-' operator when defining
a sequence of expressions in an <tt class="docutils literal"><span class="pre">And</span></tt> expression.</p>