parse.py 94.8 KB
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# parse.py - parser combinators for DHParser
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#
# Copyright 2016  by Eckhart Arnold (arnold@badw.de)
#                 Bavarian Academy of Sciences an Humanities (badw.de)
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.  See the License for the specific language governing
# permissions and limitations under the License.
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"""
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Module ``parse`` contains the python classes and functions for
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DHParser's packrat-parser. It's central class is the
``Grammar``-class, which is the base class for any concrete
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Grammar. Grammar-objects are callable and parsing is done by
calling a Grammar-object with a source text as argument.

The different parsing functions are callable descendants of class
``Parser``. Usually, they are organized in a tree and defined
within the namespace of a grammar-class. See ``ebnf.EBNFGrammar``
for an example.
"""

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from collections import defaultdict
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import copy
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from DHParser.error import Error, linebreaks, line_col
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from DHParser.log import is_logging, HistoryRecord
from DHParser.preprocess import BEGIN_TOKEN, END_TOKEN, RX_TOKEN_NAME
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from DHParser.stringview import StringView, EMPTY_STRING_VIEW
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from DHParser.syntaxtree import Node, FrozenNode, RootNode, WHITESPACE_PTYPE, \
    TOKEN_PTYPE, ZOMBIE_TAG, ResultType
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from DHParser.toolkit import sane_parser_name, escape_control_characters, get_config_value,\
    CONFIG_PRESET, re, typing, cython
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from typing import Callable, cast, List, Tuple, Set, Dict, DefaultDict, Union, Optional, Any
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__all__ = ('Parser',
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           'UnknownParserError',
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           'Grammar',
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           'EMPTY_NODE',
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           'PreprocessorToken',
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           'Token',
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           'DropToken',
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           'RegExp',
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           'RE',
           'TKN',
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           'Whitespace',
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           'DropWhitespace',
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           'mixin_comment',
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           'MetaParser',
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           'UnaryParser',
           'NaryParser',
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           'Synonym',
           'Option',
           'ZeroOrMore',
           'OneOrMore',
           'Series',
           'Alternative',
           'AllOf',
           'SomeOf',
           'Unordered',
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           'Required',
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           'Lookahead',
           'NegativeLookahead',
           'Lookbehind',
           'NegativeLookbehind',
           'last_value',
           'counterpart',
           'accumulate',
           'Capture',
           'Retrieve',
           'Pop',
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           'Forward')
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########################################################################
#
# Presets
#
########################################################################
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CONFIG_PRESET['flatten_tree_while_parsing'] = True
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########################################################################
#
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# Parser base class
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#
########################################################################


LEFT_RECURSION_DEPTH = 8  # type: int
# because of python's recursion depth limit, this value ought not to be
# set too high. PyPy allows higher values than CPython
MAX_DROPOUTS = 3  # type: int
# stop trying to recover parsing after so many errors
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EMPTY_NODE = FrozenNode(':EMPTY__', '')
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class ParserError(Exception):
    """
    A `ParserError` is thrown for those parser errors that allow the
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    controlled re-entrance of the parsing process after the error occurred.
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    If a reentry-rule has been configured for the parser where the error
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    occurred, the parser guard can resume the parsing process.
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    Currently, the only case when a `ParserError` is thrown (and not some
    different kind of error like `UnknownParserError`, is when a `Series`-
    detects a missing mandatory element.
    """
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    def __init__(self, node: Node, rest: StringView, first_throw: bool):
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        self.node = node  # type: Node
        self.rest = rest  # type: StringView
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        self.first_throw = first_throw  # type: bool
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ResumeList = List[Union[str, Any]]  # list of strings or regular expressiones
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def reentry_point(rest: StringView, rules: ResumeList) -> int:
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    """
    Finds the point where parsing should resume after a ParserError has been caught.
    Args:
        rest:  The rest of the parsed text or, in other words, the point where
                a ParserError was thrown.
        rules: A list of strings or regular expressions. The rest of the text is
                searched for each of these. The closest match is the point where
                parsing will be resumed.
    Returns:
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        The integer index of the closest reentry point or -1 if no reentry-point
        was found.
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    """
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    upper_limit = len(rest) + 1
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    i = upper_limit
    #find closest match
    for rule in rules:
        if isinstance(rule, str):
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            k = rest.find(rule)
            i = min(k if k >= 0 else upper_limit, i)
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        else:
            m = rest.search(rule)
            if m:
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                i = min(rest.index(m.start()), i)
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    # in case no rule matched return -1
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    if i == upper_limit:
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        i = -1
    return i
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ApplyFunc = Callable[['Parser'], None]
FlagFunc = Callable[[ApplyFunc, Set[ApplyFunc]], bool]


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class Parser:
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    """
    (Abstract) Base class for Parser combinator parsers. Any parser
    object that is actually used for parsing (i.e. no mock parsers)
    should should be derived from this class.

    Since parsers can contain other parsers (see classes UnaryOperator
    and NaryOperator) they form a cyclical directed graph. A root
    parser is a parser from which all other parsers can be reached.
    Usually, there is one root parser which serves as the starting
    point of the parsing process. When speaking of "the root parser"
    it is this root parser object that is meant.

    There are two different types of parsers:

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    1. *Named parsers* for which a name is set in field `parser.pname`.
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       The results produced by these parsers can later be retrieved in
       the AST by the parser name.

    2. *Anonymous parsers* where the name-field just contains the empty
       string. AST-transformation of Anonymous parsers can be hooked
       only to their class name, and not to the individual parser.

    Parser objects are callable and parsing is done by calling a parser
    object with the text to parse.

    If the parser matches it returns a tuple consisting of a node
    representing the root of the concrete syntax tree resulting from the
    match as well as the substring `text[i:]` where i is the length of
    matched text (which can be zero in the case of parsers like
    `ZeroOrMore` or `Option`). If `i > 0` then the parser has "moved
    forward".

    If the parser does not match it returns `(None, text). **Note** that
    this is not the same as an empty match `("", text)`. Any empty match
    can for example be returned by the `ZeroOrMore`-parser in case the
    contained parser is repeated zero times.

    Attributes and Properties:
        visited:  Mapping of places this parser has already been to
                during the current parsing process onto the results the
                parser returned at the respective place. This dictionary
                is used to implement memoizing.

        recursion_counter:  Mapping of places to how often the parser
                has already been called recursively at this place. This
                is needed to implement left recursion. The number of
                calls becomes irrelevant once a resault has been memoized.

        cycle_detection:  The apply()-method uses this variable to make
                sure that one and the same function will not be applied
                (recursively) a second time, if it has already been
                applied to this parser.

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        _grammar:  A reference to the Grammar object to which the parser
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                is attached.
    """

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    def __init__(self) -> None:
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        # assert isinstance(name, str), str(name)
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        self.pname = ''  # type: str
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        self.tag_name = self.ptype  # type: str
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        try:
            self._grammar = GRAMMAR_PLACEHOLDER  # type: Grammar
        except NameError:
            pass
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        self.reset()

    def __deepcopy__(self, memo):
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        """        Deepcopy method of the parser. Upon instantiation of a Grammar-
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        object, parsers will be deep-copied to the Grammar object. If a
        derived parser-class changes the signature of the constructor,
        `__deepcopy__`-method must be replaced (i.e. overridden without
        calling the same method from the superclass) by the derived class.
        """
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        duplicate = self.__class__()
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        duplicate.pname = self.pname
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        duplicate.tag_name = self.tag_name
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        return duplicate
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    def __repr__(self):
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        return self.pname + self.ptype
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    def __str__(self):
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        return self.pname + (' = ' if self.pname else '') + repr(self)
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    @property
    def ptype(self) -> str:
        """Returns a type name for the parser. By default this is the name of
        the parser class with an added leading colon ':'. """
        return ':' + self.__class__.__name__

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    @property
    def repr(self) -> str:
        """Returns the parser's name if it has a name and self.__repr___() otherwise."""
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        return self.pname if self.pname else self.__repr__()
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    def reset(self):
        """Initializes or resets any parser variables. If overwritten,
        the `reset()`-method of the parent class must be called from the
        `reset()`-method of the derived class."""
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        self.visited = dict()  # type: Dict[int, Tuple[Optional[Node], StringView]]
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        self.recursion_counter = defaultdict(int)  # type: DefaultDict[int, int]
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        self.cycle_detection = set()  # type: Set[ApplyFunc]
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    @cython.locals(location=cython.int, gap=cython.int, i=cython.int)
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    def __call__(self: 'Parser', text: StringView) -> Tuple[Optional[Node], StringView]:
        """Applies the parser to the given text. This is a wrapper method that adds
        the business intelligence that is common to all parsers. The actual parsing is
        done in the overridden method `_parse()`.
        """
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        grammar = self._grammar
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        location = grammar.document_length__ - len(text)

        try:
            if grammar.last_rb__loc__ >= location:
                grammar.rollback_to__(location)

            # if location has already been visited by the current parser,
            # return saved result
            if location in self.visited:
                # no history recording in case of memoized results
                return self.visited[location]

            # break left recursion at the maximum allowed depth
            if grammar.left_recursion_handling__:
                if self.recursion_counter[location] > LEFT_RECURSION_DEPTH:
                    grammar.recursion_locations__.add(location)
                    return None, text
                self.recursion_counter[location] += 1

            if grammar.history_tracking__:
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                grammar.call_stack__.append(self.repr if self.tag_name in (':RegExp', ':Token')
                                            else self.tag_name)
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                grammar.moving_forward__ = True

            try:
                # PARSER CALL: run _parse() method
                node, rest = self._parse(text)
            except ParserError as error:
                # does this play well with variable setting? add rollback clause here? tests needed...
                gap = len(text) - len(error.rest)
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                rules = grammar.resume_rules__.get(self.pname, [])
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                rest = error.rest[len(error.node):]
                i = reentry_point(rest, rules)
                if i >= 0 or self == grammar.start_parser__:
                    # apply reentry-rule or catch error at root-parser
                    if i < 0:
                        i = 1
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                    nd = Node(ZOMBIE_TAG, rest[:i])
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                    rest = rest[i:]
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                    assert error.node.children or (not error.node.result)
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                    if error.first_throw:
                        node = error.node
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                        node.result = node.children + (nd,)
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                    else:
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                        # TODO: ggf. Fehlermeldung, die sagt, wo es weitergeht anfügen
                        #       dürfte allerdings erst an den nächsten(!) Knoten angehängt werden (wie?)
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                        node = Node(self.tag_name, (Node(ZOMBIE_TAG, text[:gap]), error.node, nd))
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                elif error.first_throw:
                    raise ParserError(error.node, error.rest, first_throw=False)
                else:
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                    result = (Node(ZOMBIE_TAG, text[:gap]), error.node) if gap else error.node  # type: ResultType
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                    if grammar.tree__.errors[-1].code == Error.MANDATORY_CONTINUATION_AT_EOF:  # EXPERIMENTAL!!
                        node = error.node
                    else:
                        raise ParserError(Node(self.tag_name, result), text, first_throw=False)

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            if grammar.left_recursion_handling__:
                self.recursion_counter[location] -= 1
                # don't clear recursion_locations__ !!!

            if node is None:
                # retrieve an earlier match result (from left recursion) if it exists
                if location in grammar.recursion_locations__:
                    if location in self.visited:
                        node, rest = self.visited[location]
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                        if location != grammar.last_recursion_location__:
                            grammar.tree__.add_error(node, Error("Left recursion encountered. "
                                                                 "Refactor grammar to avoid slow parsing.",
                                                                 node.pos, Error.LEFT_RECURSION_WARING))
                            grammar.last_recursion_location__ = location
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                    # don't overwrite any positive match (i.e. node not None) in the cache
                    # and don't add empty entries for parsers returning from left recursive calls!
                elif grammar.memoization__:
                    # otherwise also cache None-results
                    self.visited[location] = (None, rest)
            else:
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                # assert node._pos < 0 or node == EMPTY_NODE
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                node._pos = location
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                # assert node._pos >= 0 or node == EMPTY_NODE, \
                #     str("%i < %i" % (grammar.document_length__, location))
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                if (grammar.last_rb__loc__ < location
                        and (grammar.memoization__ or location in grammar.recursion_locations__)):
                    # - variable manipulating parsers will not be entered into the cache,
                    #   because caching would interfere with changes of variable state
                    # - in case of left recursion, the first recursive step that
                    #   matches will store its result in the cache
                    # TODO: need a unit-test concerning interference of variable manipulation and left recursion algorithm?
                    self.visited[location] = (node, rest)

            # Mind that memoized parser calls will not appear in the history record!
            # Does this make sense? Or should it be changed?
            if grammar.history_tracking__:
                # don't track returning parsers except in case an error has occurred
                # remaining = len(rest)
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                if grammar.moving_forward__:
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                    record = HistoryRecord(grammar.call_stack__, node or EMPTY_NODE, text,
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                                           grammar.line_col__(text))
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                    grammar.history__.append(record)
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                elif node:
                    nid = id(node)  # type: int
                    if nid in grammar.tree__.error_nodes:
                        record = HistoryRecord(grammar.call_stack__, node or EMPTY_NODE, text,
                                               grammar.line_col__(text),
                                               grammar.tree__.error_nodes[nid])
                        grammar.history__.append(record)
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                grammar.moving_forward__ = False
                grammar.call_stack__.pop()

        except RecursionError:
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            node = Node(ZOMBIE_TAG, str(text[:min(10, max(1, text.find("\n")))]) + " ...")
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            node._pos = location
            grammar.tree__.new_error(node, "maximum recursion depth of parser reached; "
                                           "potentially due to too many errors!")
            rest = EMPTY_STRING_VIEW

        return node, rest
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    def __add__(self, other: 'Parser') -> 'Series':
        """The + operator generates a series-parser that applies two
        parsers in sequence."""
        return Series(self, other)

    def __or__(self, other: 'Parser') -> 'Alternative':
        """The | operator generates an alternative parser that applies
        the first parser and, if that does not match, the second parser.
        """
        return Alternative(self, other)

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    def _parse(self, text: StringView) -> Tuple[Optional[Node], StringView]:
        """Applies the parser to the given `text` and returns a node with
        the results or None as well as the text at the position right behind
        the matching string."""
        raise NotImplementedError

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    @property
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    def grammar(self) -> 'Grammar':
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        try:
            if self._grammar != GRAMMAR_PLACEHOLDER:
                return self._grammar
            else:
                raise AssertionError('Grammar has not yet been set!')
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        except (AttributeError, NameError):
            raise AttributeError('Parser placeholder does not have a grammar!')
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    @grammar.setter
    def grammar(self, grammar: 'Grammar'):
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        try:
            if self._grammar == GRAMMAR_PLACEHOLDER:
                self._grammar = grammar
                # self._grammar_assigned_notifier()
            elif self._grammar != grammar:
                  raise AssertionError("Parser has already been assigned"
                                       "to a different Grammar object!")
        except AttributeError:
            pass  # ignore setting of grammar attribute for placeholder parser
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        except NameError:  # Cython: No access to GRAMMA_PLACEHOLDER, yet :-(
            self._grammar = grammar
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    def _apply(self, func: ApplyFunc, flip: FlagFunc) -> bool:
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        """
        Applies function `func(parser)` recursively to this parser and all
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        descendant parsers, if any exist.

        In order to break cycles, function `flip` is called, which should
        return `True`, if this parser has already been visited. If not, it
        flips the cycle detection flag and returns `False`.

        This is a protected function and should not called from outside
        class Parser or any of its descendants. The entry point for external
        calls is the method `apply()` without underscore!
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        """
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        if flip(func, self.cycle_detection):
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            return False
        else:
            func(self)
            return True

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    def apply(self, func: ApplyFunc):
        """
        Applies function `func(parser)` recursively to this parser and all
        descendant parsers, if any exist. Traversal is pre-order.
        """
        def positive_flip(f: ApplyFunc, flagged: Set[Callable]) -> bool:
            """Returns True, if function `f` has already been applied to this
            parser and sets the flag accordingly. Interprets `f in flagged == True`
            as meaning that `f` has already been applied."""
            if f in flagged:
                return True
            else:
                flagged.add(f)
                return False

        def negative_flip(f: ApplyFunc, flagged: Set[Callable]) -> bool:
            """Returns True, if function `f` has already been applied to this
            parser and sets the flag accordingly. Interprets `f in flagged == False`
            as meaning that `f` has already been applied."""
            if f not in flagged:
                return True
            else:
                flagged.remove(f)
                return False

        if func in self.cycle_detection:
            self._apply(func, negative_flip)
        else:
            self._apply(func, positive_flip)

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PARSER_PLACEHOLDER = Parser()
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########################################################################
#
# Grammar class, central administration of all parser of a grammar
#
########################################################################


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def mixin_comment(whitespace: str, comment: str) -> str:
    """
    Returns a regular expression that merges comment and whitespace
    regexps. Thus comments cann occur whereever whitespace is allowed
    and will be skipped just as implicit whitespace.

    Note, that because this works on the level of regular expressions,
    nesting comments is not possible. It also makes it much harder to
    use directives inside comments (which isn't recommended, anyway).
    """
    wspc = '(?:' + whitespace + '(?:' + comment + whitespace + ')*)'
    return wspc


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class UnknownParserError(KeyError):
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    """UnknownParserError is raised if a Grammar object is called with a
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    parser that does not exist or if in the course of parsing a parser
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    is referred to that does not exist."""
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class Grammar:
    r"""
    Class Grammar directs the parsing process and stores global state
    information of the parsers, i.e. state information that is shared
    accross parsers.

    Grammars are basically collections of parser objects, which are
    connected to an instance object of class Grammar. There exist two
    ways of connecting parsers to grammar objects: Either by passing
    the root parser object to the constructor of a Grammar object
    ("direct instantiation"), or by assigning the root parser to the
    class variable "root__" of a descendant class of class Grammar.

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    Example for direct instantiation of a grammar::
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        >>> number = RE('\d+') + RE('\.') + RE('\d+') | RE('\d+')
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        >>> number_parser = Grammar(number)
        >>> number_parser("3.1416").content
        '3.1416'

    Collecting the parsers that define a grammar in a descendant class of
    class Grammar and assigning the named parsers to class variables
    rather than global variables has several advantages:

    1. It keeps the namespace clean.

    2. The parser names of named parsers do not need to be passed to the
       constructor of the Parser object explicitly, but it suffices to
       assign them to class variables, which results in better
       readability of the Python code.

    3. The parsers in the class do not necessarily need to be connected
       to one single root parser, which is helpful for testing and
       building up a parser successively of several components.

    As a consequence, though, it is highly recommended that a Grammar
    class should not define any other variables or methods with names
    that are legal parser names. A name ending with a double
    underscore '__' is *not* a legal parser name and can safely be
    used.

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    Example::
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        class Arithmetic(Grammar):
            # special fields for implicit whitespace and comment configuration
            COMMENT__ = r'#.*(?:\n|$)'  # Python style comments
            wspR__ = mixin_comment(whitespace=r'[\t ]*', comment=COMMENT__)

            # parsers
            expression = Forward()
            INTEGER = RE('\\d+')
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            factor = INTEGER | TKN("(") + expression + TKN(")")
            term = factor + ZeroOrMore((TKN("*") | TKN("/")) + factor)
            expression.set(term + ZeroOrMore((TKN("+") | TKN("-")) + term))
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            root__ = expression

    Upon instantiation the parser objects are deep-copied to the
    Grammar object and assigned to object variables of the same name.
    Any parser that is directly assigned to a class variable is a
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    'named' parser and its field `parser.pname` contains the variable
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    name after instantiation of the Grammar class. All other parsers,
    i.e. parsers that are defined within a `named` parser, remain
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    "anonymous parsers" where `parser.pname` is the empty string, unless
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    a name has been passed explicitly upon instantiation.
    If one and the same parser is assigned to several class variables
    such as, for example the parser `expression` in the example above,
    the first name sticks.

    Grammar objects are callable. Calling a grammar object with a UTF-8
    encoded document, initiates the parsing of the document with the
    root parser. The return value is the concrete syntax tree. Grammar
    objects can be reused (i.e. called again) after parsing. Thus, it
    is not necessary to instantiate more than one Grammar object per
    thread.

    Grammar classes contain a few special class fields for implicit
    whitespace and comments that should be overwritten, if the defaults
    (no comments, horizontal right aligned whitespace) don't fit:

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    Class Attributes:
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        root__:  The root parser of the grammar. Theoretically, all parsers of the
                 grammar should be reachable by the root parser. However, for testing
                 of yet incomplete grammars class Grammar does not assume that this
                 is the case.
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        resume_rules__: A mapping of parser names to a list of regular expressions or search
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                strings that act as rules to find the the reentry point if a ParserError
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                was thrown during the execution of the parser with the respective name.

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        parser_initializiation__:  Before the parser class (!) has been initialized,
                 which happens upon the first time it is instantiated (see
                 :func:_assign_parser_names()` for an explanation), this class
                 field contains a value other than "done". A value of "done" indicates
                 that the class has already been initialized.
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        python__src__:  For the purpose of debugging and inspection, this field can
                 take the python src of the concrete grammar class
                 (see `dsl.grammar_provider`).

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    Instance Attributes:
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        all_parsers__:  A set of all parsers connected to this grammar object

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        start_parser__:  During parsing, the parser with which the parsing process
                was started (see method `__call__`) or `None` if no parsing process
                is running.

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        history_tracking__:  A flag indicating that the parsing history shall
                be tracked

        _dirty_flag__:  A flag indicating that the Grammar has been called at
                least once so that the parsing-variables need to be reset
                when it is called again.

        document__:  the text that has most recently been parsed or that is
                currently being parsed.

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        document_length__:  the length of the document.

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        document_lbreaks__:  list of linebreaks within the document, starting
                with -1 and ending with EOF. This helps generating line
                and column number for history recording and will only be
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                initialized if :attr:`history_tracking__` is true.
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        tree__: The root-node of the parsing tree. This variable is available
               for error-reporting already during parsing  via
               ``self.grammar.tree__.add_error``, but it references the full
               parsing tree only after parsing has been finished.

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        _reversed__:  the same text in reverse order - needed by the `Lookbehind`-
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                parsers.

        variables__:  A mapping for variable names to a stack of their respective
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                string values - needed by the :class:`Capture`-, :class:`Retrieve`-
                and :class:`Pop`-parsers.
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        rollback__:  A list of tuples (location, rollback-function) that are
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                deposited by the :class:`Capture`- and :class:`Pop`-parsers.
                If the parsing process reaches a dead end then all
                rollback-functions up to the point to which it retreats will be
                called and the state of the variable stack restored accordingly.
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        last_rb__loc__:  The last, i.e. most advanced location in the text
                where a variable changing operation occurred. If the parser
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                backtracks to a location at or before last_rb__loc__ (i.e.
                location <= last_rb__loc__) then a rollback of all variable
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                changing operations is necessary that occurred after the
                location to which the parser backtracks. This is done by
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                calling method :func:`rollback_to__(location)`.
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        call_stack__:  A stack of the tag names of all parsers that have been called.
                This is required for recording the parser history (for debugging)
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                and, eventually, i.e. one day in the future, for tracing through
                the parsing process.

        history__:  A list of parser-call-stacks. A parser-call-stack is
                appended to the list each time a parser either matches, fails
                or if a parser-error occurs.

        moving_forward__: This flag indicates that the parsing process is currently
                moving forward . It is needed to reduce noise in history recording
                and should not be considered as having a valid value if history
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                recording is turned off! (See :func:`add_parser_guard` and its local
                function :func:`guarded_call`)
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        recursion_locations__:  Stores the locations where left recursion was
                detected. Needed to provide minimal memoization for the left
                recursion detection algorithm, but, strictly speaking, superfluous
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                if full memoization is enabled. (See :func:`Parser.__call__()`)

        last_recursion_location__:  Last location where left recursion was
                detected. This is used to avoid reduplicating warning messages
                about left recursion.
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        memoization__:  Turns full memoization on or off. Turning memoization off
                results in less memory usage and sometimes reduced parsing time.
                In some situations it may drastically increase parsing time, so
                it is safer to leave it on. (Default: on)

        left_recursion_handling__:  Turns left recursion handling on or off.
                If turned off, a recursion error will result in case of left
                recursion.
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        flatten_tree__:  If True (default), anonymous nodes will be flattened
                during parsing already. This greatly reduces the concrete syntax
                tree and simplifies and speeds up abstract syntax tree generation.
                The initial value will be read from the config variable
                'flatten_tree_while_parsing' upon class instantiation.
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    """
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    python_src__ = ''  # type: str
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    root__ = PARSER_PLACEHOLDER  # type: Parser
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    # root__ must be overwritten with the root-parser by grammar subclass
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    parser_initialization__ = ["pending"]  # type: List[str]
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    resume_rules__ = dict()  # type: Dict[str, ResumeList]
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    # some default values
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    # COMMENT__ = r''  # type: str  # r'#.*(?:\n|$)'
    # WSP_RE__ = mixin_comment(whitespace=r'[\t ]*', comment=COMMENT__)  # type: str
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    @classmethod
    def _assign_parser_names__(cls):
        """
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        Initializes the `parser.pname` fields of those
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        Parser objects that are directly assigned to a class field with
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        the field's name, e.g.::
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            class Grammar(Grammar):
                ...
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                symbol = RE('(?!\\d)\\w+')
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        After the call of this method symbol.pname == "symbol" holds.
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        Parser names starting or ending with a double underscore like
        ``root__`` will be ignored. See :func:`sane_parser_name()`
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        This is done only once, upon the first instantiation of the
        grammar class!

        Attention: If there exists more than one reference to the same
        parser, only the first one will be chosen for python versions
        greater or equal 3.6.  For python version <= 3.5 an arbitrarily
        selected reference will be chosen. See PEP 520
        (www.python.org/dev/peps/pep-0520/) for an explanation of why.
        """
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        if cls.parser_initialization__[0] != "done":
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            cdict = cls.__dict__
            for entry, parser in cdict.items():
                if isinstance(parser, Parser) and sane_parser_name(entry):
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                    if isinstance(parser, Forward):
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                        if not cast(Forward, parser).parser.pname:
                            cast(Forward, parser).parser.pname = entry
                    else:   # if not parser.pname:
                        parser.pname = entry
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            cls.parser_initialization__[0] = "done"
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    def __init__(self, root: Parser = None) -> None:
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        self.all_parsers__ = set()             # type: Set[Parser]
        self.start_parser__ = None             # type: Optional[Parser]
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        self._dirty_flag__ = False             # type: bool
        self.history_tracking__ = False        # type: bool
        self.memoization__ = True              # type: bool
        self.left_recursion_handling__ = True  # type: bool
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        self.flatten_tree__ = get_config_value('flatten_tree_while_parsing')  # type: bool
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        self._reset__()

        # prepare parsers in the class, first
        self._assign_parser_names__()

        # then deep-copy the parser tree from class to instance;
        # parsers not connected to the root object will be copied later
        # on demand (see Grammar.__getitem__()). Usually, the need to
        # do so only arises during testing.
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        self.root_parser__ = copy.deepcopy(root) if root else copy.deepcopy(self.__class__.root__)
        self.root_parser__.apply(self._add_parser__)
        assert 'root_parser__' in self.__dict__
        assert self.root_parser__ == self.__dict__['root_parser__']
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    def __getitem__(self, key):
        try:
            return self.__dict__[key]
        except KeyError:
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            parser_template = getattr(self.__class__, key, None)
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            if parser_template:
                # add parser to grammar object on the fly...
                parser = copy.deepcopy(parser_template)
                parser.apply(self._add_parser__)
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                assert self[key] == parser
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                return self[key]
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            raise UnknownParserError('Unknown parser "%s" !' % key)
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    def __contains__(self, key):
        return key in self.__dict__ or hasattr(self, key)
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    def _reset__(self):
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        self.tree__ = RootNode()              # type: RootNode
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        self.document__ = EMPTY_STRING_VIEW   # type: StringView
        self._reversed__ = EMPTY_STRING_VIEW  # type: StringView
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        self.document_length__ = 0            # type: int
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        self.document_lbreaks__ = []          # type: List[int]
        # variables stored and recalled by Capture and Retrieve parsers
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        self.variables__ = defaultdict(lambda: [])  # type: DefaultDict[str, List[str]]
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        self.rollback__ = []                  # type: List[Tuple[int, Callable]]
        self.last_rb__loc__ = -1              # type: int
        # support for call stack tracing
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        self.call_stack__ = []                # type: List[str]
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        # snapshots of call stacks
        self.history__ = []                   # type: List[HistoryRecord]
        # also needed for call stack tracing
        self.moving_forward__ = False         # type: bool
        self.recursion_locations__ = set()    # type: Set[int]
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        self.last_recursion_location__ = -1   # type: int
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    @property
    def reversed__(self) -> StringView:
        """
        Returns a reversed version of the currently parsed document. As
        about the only case where this is needed is the Lookbehind-parser,
        this is done lazily.
        """
        if not self._reversed__:
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            self._reversed__ = StringView(self.document__.get_text()[::-1])
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        return self._reversed__


    def _add_parser__(self, parser: Parser) -> None:
        """
        Adds the particular copy of the parser object to this
        particular instance of Grammar.
        """
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        if parser.pname:
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            # prevent overwriting instance variables or parsers of a different class
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            assert parser.pname not in self.__dict__ or \
                   isinstance(self.__dict__[parser.pname], parser.__class__), \
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                ('Cannot add parser "%s" because a field with the same name '
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                 'already exists in grammar object: %s!'
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                 % (parser.pname, str(self.__dict__[parser.pname])))
            setattr(self, parser.pname, parser)
        parser.tag_name = parser.pname or parser.ptype
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        self.all_parsers__.add(parser)
        parser.grammar = self


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    def __call__(self,
                 document: str,
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                 start_parser: Union[str, Parser] = "root_parser__",
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                 track_history: bool = False) -> RootNode:
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        """
        Parses a document with with parser-combinators.

        Args:
            document (str): The source text to be parsed.
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            start_parser (str or Parser): The name of the parser with which
                to start. This is useful for testing particular parsers
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                (i.e. particular parts of the EBNF-Grammar.)
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            track_history (bool): If true, the parsing history will be
                recorded in self.history__. If logging is turned on (i.e.
                DHParser.log.is_logging() returns true), the parsing history
                will always be recorded, even if `False` is passed to
                the `track_history` parameter.
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        Returns:
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            Node: The root node to the parse tree.
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        """
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        def tail_pos(predecessors: Union[List[Node], Tuple[Node, ...], None]) -> int:
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            """Adds the position after the last node in the list of
            predecessors to the node."""
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            return predecessors[-1].pos + len(predecessors[-1]) if predecessors else 0
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        # assert isinstance(document, str), type(document)
        if self._dirty_flag__:
            self._reset__()
            for parser in self.all_parsers__:
                parser.reset()
        else:
            self._dirty_flag__ = True
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        self.history_tracking__ = track_history or is_logging()
        # safe tracking state, because history_tracking__ might be set to false, later,
        # but original tracking state is needed for additional error information.
        track_history = self.history_tracking__
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        self.document__ = StringView(document)
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        self.document_length__ = len(self.document__)
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        self.document_lbreaks__ = linebreaks(document) if self.history_tracking__ else []
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        self.last_rb__loc__ = -1  # rollback location
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        parser = self[start_parser] if isinstance(start_parser, str) else start_parser
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        self.start_parser__ = parser
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        assert parser.grammar == self, "Cannot run parsers from a different grammar object!" \
                                       " %s vs. %s" % (str(self), str(parser.grammar))
        result = None  # type: Optional[Node]
        stitches = []  # type: List[Node]
        rest = self.document__
        if not rest:
            result, _ = parser(rest)
            if result is None:
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                result = Node(ZOMBIE_TAG, '').with_pos(0)
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                self.tree__.new_error(result,
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                                      'Parser "%s" did not match empty document.' % str(parser),
                                      Error.PARSER_DID_NOT_MATCH)
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        while rest and len(stitches) < MAX_DROPOUTS:
            result, rest = parser(rest)
            if rest:
                fwd = rest.find("\n") + 1 or len(rest)
                skip, rest = rest[:fwd], rest[fwd:]
                if result is None:
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                    err_info = '' if not track_history else \
                               '\n    Most advanced: %s\n    Last match:    %s;' % \
                               (str(HistoryRecord.most_advanced_match(self.history__)),
                                str(HistoryRecord.last_match(self.history__)))
                    # Check if a Lookahead-Parser did match. Needed for testing, because
                    # in a test case this is not necessarily an error.
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                    last_record = self.history__[-2] if len(self.history__) > 1 else None  # type: Optional[HistoryRecord]
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                    if last_record and parser != self.root_parser__ \
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                            and last_record.status == HistoryRecord.MATCH \
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                            and last_record.node.pos \
                            + len(last_record.node) >= len(self.document__) \
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                            and any(tn in self and isinstance(self[tn], Lookahead)
                                    or tn[0] == ':' and issubclass(eval(tn[1:]), Lookahead)
                                    for tn in last_record.call_stack):
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                        error_msg = 'Parser did not match except for lookahead! ' + err_info
                        error_code = Error.PARSER_LOOKAHEAD_MATCH_ONLY
                    else:
                        error_msg = 'Parser did not match!' + err_info
                        error_code = Error.PARSER_DID_NOT_MATCH
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                else:
                    stitches.append(result)
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                    error_msg = "Parser stopped before end" \
                        + (("! trying to recover"
                            + (" but stopping history recording at this point."
                               if self.history_tracking__ else "..."))
                            if len(stitches) < MAX_DROPOUTS
                            else " too often! Terminating parser.")
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                    error_code = Error.PARSER_STOPPED_BEFORE_END
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                stitches.append(Node(ZOMBIE_TAG, skip).with_pos(tail_pos(stitches)))
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                self.tree__.new_error(stitches[-1], error_msg, error_code)
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                if self.history_tracking__:
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                    # # some parsers may have matched and left history records with nodes != None.
                    # # Because these are not connected to the stitched root node, their pos-
                    # # properties will not be initialized by setting the root node's pos property
                    # # to zero. Therefore, their pos properties need to be initialized here
                    # for record in self.history__:
                    #     if record.node and record.node._pos < 0:
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                    #         record.node.with_pos(0)
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                    record = HistoryRecord(self.call_stack__.copy(), stitches[-1], rest,
                                           self.line_col__(rest))
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                    self.history__.append(record)
                    # stop history tracking when parser returned too early
                    self.history_tracking__ = False
        if stitches:
            if rest:
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                stitches.append(Node(ZOMBIE_TAG, rest))
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            #try:
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            result = Node(ZOMBIE_TAG, tuple(stitches)).with_pos(0)
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            # except AssertionError as error:
            #     # some debugging output
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            #     print(Node(ZOMBIE_TAG, tuple(stitches)).as_sxpr())
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            #     raise error
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        if any(self.variables__.values()):
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            error_msg = "Capture-retrieve-stack not empty after end of parsing: " \
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                + str(self.variables__)
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            error_code = Error.CAPTURE_STACK_NOT_EMPTY
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            if result:
                if result.children:
                    # add another child node at the end to ensure that the position
                    # of the error will be the end of the text. Otherwise, the error
                    # message above ("...after end of parsing") would appear illogical.
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                    error_node = Node(ZOMBIE_TAG, '').with_pos(tail_pos(result.children))
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                    self.tree__.new_error(error_node, error_msg, error_code)
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                    result.result = result.children + (error_node,)
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                else:
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                    self.tree__.new_error(result, error_msg, error_code)
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        # result.pos = 0  # calculate all positions
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        # result.errors(self.document__)
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        if result:
            self.tree__.swallow(result)
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        self.start_parser__ = None
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        return self.tree__
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    def push_rollback__(self, location, func):
        """
        Adds a rollback function that either removes or re-adds
        values on the variable stack (`self.variables`) that have been
        added (or removed) by Capture or Pop Parsers, the results of
        which have been dismissed.
        """
        self.rollback__.append((location, func))
        self.last_rb__loc__ = location


    def rollback_to__(self, location):
        """
        Rolls back the variable stacks (`self.variables`) to its
        state at an earlier location in the parsed document.
        """
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        while self.rollback__ and self.rollback__[-1][0] >= location:
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            _, rollback_func = self.rollback__.pop()
            # assert not loc > self.last_rb__loc__, \
            #     "Rollback confusion: line %i, col %i < line %i, col %i" % \
            #     (*line_col(self.document__, len(self.document__) - loc),
            #      *line_col(self.document__, len(self.document__) - self.last_rb__loc__))
            rollback_func()
        self.last_rb__loc__ == self.rollback__[-1][0] if self.rollback__ \
            else (len(self.document__) + 1)


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    def line_col__(self, text):
        """
        Returns the line and column where text is located in the document.
        Does not check, whether text is actually a substring of the currently
        parsed document.
        """
        return line_col(self.document_lbreaks__, self.document_length__ - len(text))


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def dsl_error_msg(parser: Parser, error_str: str) -> str:
    """
    Returns an error message for errors in the parser configuration,
    e.g. errors that result in infinite loops.

    Args:
        parser (Parser):  The parser where the error was noticed. Note
            that this is not necessarily the parser that caused the
            error but only where the error became apparent.
        error_str (str):  A short string describing the error.
    Returns:
        str: An error message including the call stack if history
        tacking has been turned in the grammar object.
    """
    msg = ["DSL parser specification error:", error_str, 'Caught by parser "%s".' % str(parser)]
    if parser.grammar.history__:
        msg.extend(["\nCall stack:", parser.grammar.history__[-1].stack])
    else:
        msg.extend(["\nEnable history tracking in Grammar object to display call stack."])
    return " ".join(msg)


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GRAMMAR_PLACEHOLDER = Grammar()
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########################################################################
#
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# _Token and Regular Expression parser classes (i.e. leaf classes)
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#
########################################################################


class PreprocessorToken(Parser):
    """
    Parses tokens that have been inserted by a preprocessor.

    Preprocessors can generate Tokens with the ``make_token``-function.
    These tokens start and end with magic characters that can only be
    matched by the PreprocessorToken Parser. Such tokens can be used to
    insert BEGIN - END delimiters at the beginning or ending of a
    quoted block, for example.
    """

    def __init__(self, token: str) -> None:
        assert token and token.isupper()
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        assert RX_TOKEN_NAME.match(token)
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        super(PreprocessorToken, self).__init__()
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        self.pname = token
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    def __deepcopy__(self, memo):
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        duplicate = self.__class__(self.pname)
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        # duplicate.pname = self.pname  # will be written by the constructor, anyway
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        duplicate.tag_name = self.tag_name
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        return duplicate

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    def _parse(self, text: StringView) -> Tuple[Optional[Node], StringView]:
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        if text[0:1] == BEGIN_TOKEN:
            end = text.find(END_TOKEN, 1)
            if end < 0:
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                node = Node(self.tag_name, '')
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                self.grammar.tree__.new_error(
                    node,
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                    'END_TOKEN delimiter missing from preprocessor token. '
                    '(Most likely due to a preprocessor bug!)')  # type: Node
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                return node, text[1:]
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            elif end == 0:
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                node = Node(self.tag_name, '')
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                self.grammar.tree__.new_error(
                    node,
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                    'Preprocessor-token cannot have zero length. '
                    '(Most likely due to a preprocessor bug!)')
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                return node, text[2:]
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            elif text.find(BEGIN_TOKEN, 1, end) >= 0:
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                node = Node(self.tag_name, text[len(self.pname) + 1:end])
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                self.grammar.tree__.new_error(
                    node,
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                    'Preprocessor-tokens must not be nested or contain '
                    'BEGIN_TOKEN delimiter as part of their argument. '
                    '(Most likely due to a preprocessor bug!)')
                return node, text[end:]
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            if text[1:len(self.pname) + 1] == self.pname:
                return Node(self.tag_name, text[len(self.pname) + 2:end]), text[end + 1:]
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        return None, text


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class Token(Parser):
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    """
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    Parses plain text strings. (Could be done by RegExp as well, but is faster.)
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    Example::

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        >>> while_token = Token("while")
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        >>> Grammar(while_token)("while").content
        'while'
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    """
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    assert TOKEN_PTYPE == ":Token"
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    def __init__(self, text: str) -> None:
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        super(Token, self).__init__()
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        self.text = text
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        self.len = len(text)
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    def __deepcopy__(self, memo):
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        duplicate = self.__class__(self.text)
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        duplicate.pname = self.pname
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        duplicate.tag_name = self.tag_name
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        return duplicate
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    def _parse(self, text: StringView) -> Tuple[Optional[Node], StringView]:
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        if text.startswith(self.text):
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            return Node(self.tag_name, self.text, True), text[self.len:]
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        return None, text

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    def __repr__(self):
        return ("'%s'" if self.text.find("'") <= 0 else '"%s"') % self.text

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class DropToken(Token):
    """
    Parses play text string, but returns EMPTY_NODE rather than the parsed
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    string on a match. Violates the invariant: str(parse(text)) == text !
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    """
    def _parse(self, text: StringView) -> Tuple[Optional[Node], StringView]:
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        assert not self.pname, "DropToken must not be used for named parsers!"
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        if text.startswith(self.text):
            return EMPTY_NODE, text[self.len:]
            # return Node(self.tag_name, self.text, True), text[self.len:]
        return None, text


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class RegExp(Parser):
    r"""
    Regular expression parser.

    The RegExp-parser parses text that matches a regular expression.
    RegExp can also be considered as the "atomic parser", because all
    other parsers delegate part of the parsing job to other parsers,
    but do not match text directly.

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    Example::

        >>> word = RegExp(r'\w+')
        >>> Grammar(word)("Haus").content
        'Haus'
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    EBNF-Notation:  ``/ ... /``
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    EBNF-Example:   ``word = /\w+/``
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    """

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    def