324 lines
12 KiB
Python
324 lines
12 KiB
Python
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"""
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Functions for reversing a regular expression (used in reverse URL resolving).
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Used internally by Django and not intended for external use.
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This is not, and is not intended to be, a complete reg-exp decompiler. It
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should be good enough for a large class of URLS, however.
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"""
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# Mapping of an escape character to a representative of that class. So, e.g.,
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# "\w" is replaced by "x" in a reverse URL. A value of None means to ignore
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# this sequence. Any missing key is mapped to itself.
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ESCAPE_MAPPINGS = {
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"A": None,
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"b": None,
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"B": None,
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"d": '0',
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"D": "x",
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"s": " ",
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"S": "x",
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"w": "x",
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"W": "!",
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"Z": None,
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}
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class Choice(list):
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"""
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Used to represent multiple possibilities at this point in a pattern string.
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We use a distinguished type, rather than a list, so that the usage in the
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code is clear.
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"""
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class Group(list):
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"""
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Used to represent a capturing group in the pattern string.
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"""
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class NonCapture(list):
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"""
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Used to represent a non-capturing group in the pattern string.
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"""
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def normalize(pattern):
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"""
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Given a reg-exp pattern, normalizes it to a list of forms that suffice for
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reverse matching. This does the following:
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(1) For any repeating sections, keeps the minimum number of occurrences
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permitted (this means zero for optional groups).
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(2) If an optional group includes parameters, include one occurrence of
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that group (along with the zero occurrence case from step (1)).
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(3) Select the first (essentially an arbitrary) element from any character
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class. Select an arbitrary character for any unordered class (e.g. '.'
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or '\w') in the pattern.
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(5) Ignore comments and any of the reg-exp flags that won't change
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what we construct ("iLmsu"). "(?x)" is an error, however.
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(6) Raise an error on all other non-capturing (?...) forms (e.g.
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look-ahead and look-behind matches) and any disjunctive ('|')
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constructs.
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Django's URLs for forward resolving are either all positional arguments or
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all keyword arguments. That is assumed here, as well. Although reverse
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resolving can be done using positional args when keyword args are
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specified, the two cannot be mixed in the same reverse() call.
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"""
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# Do a linear scan to work out the special features of this pattern. The
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# idea is that we scan once here and collect all the information we need to
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# make future decisions.
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result = []
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non_capturing_groups = []
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consume_next = True
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pattern_iter = next_char(iter(pattern))
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num_args = 0
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# A "while" loop is used here because later on we need to be able to peek
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# at the next character and possibly go around without consuming another
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# one at the top of the loop.
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ch, escaped = pattern_iter.next()
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try:
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while True:
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if escaped:
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result.append(ch)
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elif ch == '.':
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# Replace "any character" with an arbitrary representative.
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result.append("x")
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elif ch == '|':
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# FIXME: One day we'll should do this, but not in 1.0.
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raise NotImplementedError
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elif ch == "^":
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pass
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elif ch == '$':
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break
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elif ch == ')':
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# This can only be the end of a non-capturing group, since all
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# other unescaped parentheses are handled by the grouping
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# section later (and the full group is handled there).
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#
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# We regroup everything inside the capturing group so that it
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# can be quantified, if necessary.
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start = non_capturing_groups.pop()
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inner = NonCapture(result[start:])
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result = result[:start] + [inner]
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elif ch == '[':
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# Replace ranges with the first character in the range.
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ch, escaped = pattern_iter.next()
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result.append(ch)
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ch, escaped = pattern_iter.next()
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while escaped or ch != ']':
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ch, escaped = pattern_iter.next()
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elif ch == '(':
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# Some kind of group.
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ch, escaped = pattern_iter.next()
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if ch != '?' or escaped:
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# A positional group
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name = "_%d" % num_args
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num_args += 1
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result.append(Group((("%%(%s)s" % name), name)))
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walk_to_end(ch, pattern_iter)
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else:
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ch, escaped = pattern_iter.next()
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if ch in "iLmsu#":
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# All of these are ignorable. Walk to the end of the
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# group.
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walk_to_end(ch, pattern_iter)
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elif ch == ':':
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# Non-capturing group
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non_capturing_groups.append(len(result))
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elif ch != 'P':
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# Anything else, other than a named group, is something
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# we cannot reverse.
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raise ValueError("Non-reversible reg-exp portion: '(?%s'" % ch)
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else:
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ch, escaped = pattern_iter.next()
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if ch != '<':
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raise ValueError("Non-reversible reg-exp portion: '(?P%s'" % ch)
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# We are in a named capturing group. Extra the name and
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# then skip to the end.
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name = []
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ch, escaped = pattern_iter.next()
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while ch != '>':
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name.append(ch)
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ch, escaped = pattern_iter.next()
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param = ''.join(name)
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result.append(Group((("%%(%s)s" % param), param)))
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walk_to_end(ch, pattern_iter)
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elif ch in "*?+{":
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# Quanitifers affect the previous item in the result list.
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count, ch = get_quantifier(ch, pattern_iter)
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if ch:
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# We had to look ahead, but it wasn't need to compute the
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# quanitifer, so use this character next time around the
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# main loop.
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consume_next = False
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if count == 0:
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if contains(result[-1], Group):
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# If we are quantifying a capturing group (or
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# something containing such a group) and the minimum is
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# zero, we must also handle the case of one occurrence
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# being present. All the quantifiers (except {0,0},
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# which we conveniently ignore) that have a 0 minimum
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# also allow a single occurrence.
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result[-1] = Choice([None, result[-1]])
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else:
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result.pop()
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elif count > 1:
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result.extend([result[-1]] * (count - 1))
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else:
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# Anything else is a literal.
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result.append(ch)
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if consume_next:
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ch, escaped = pattern_iter.next()
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else:
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consume_next = True
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except StopIteration:
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pass
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except NotImplementedError:
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# A case of using the disjunctive form. No results for you!
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return zip([''], [[]])
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return zip(*flatten_result(result))
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def next_char(input_iter):
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"""
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An iterator that yields the next character from "pattern_iter", respecting
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escape sequences. An escaped character is replaced by a representative of
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its class (e.g. \w -> "x"). If the escaped character is one that is
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skipped, it is not returned (the next character is returned instead).
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Yields the next character, along with a boolean indicating whether it is a
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raw (unescaped) character or not.
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"""
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for ch in input_iter:
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if ch != '\\':
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yield ch, False
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continue
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ch = input_iter.next()
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representative = ESCAPE_MAPPINGS.get(ch, ch)
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if representative is None:
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continue
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yield representative, True
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def walk_to_end(ch, input_iter):
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"""
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The iterator is currently inside a capturing group. We want to walk to the
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close of this group, skipping over any nested groups and handling escaped
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parentheses correctly.
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"""
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if ch == '(':
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nesting = 1
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else:
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nesting = 0
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for ch, escaped in input_iter:
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if escaped:
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continue
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elif ch == '(':
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nesting += 1
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elif ch == ')':
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if not nesting:
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return
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nesting -= 1
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def get_quantifier(ch, input_iter):
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"""
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Parse a quantifier from the input, where "ch" is the first character in the
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quantifier.
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Returns the minimum number of occurences permitted by the quantifier and
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either None or the next character from the input_iter if the next character
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is not part of the quantifier.
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"""
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if ch in '*?+':
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try:
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ch2, escaped = input_iter.next()
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except StopIteration:
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ch2 = None
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if ch2 == '?':
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ch2 = None
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if ch == '+':
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return 1, ch2
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return 0, ch2
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quant = []
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while ch != '}':
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ch, escaped = input_iter.next()
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quant.append(ch)
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values = ''.join(quant).split(',')
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# Consume the trailing '?', if necessary.
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try:
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ch, escaped = input_iter.next()
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except StopIteration:
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ch = None
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if ch == '?':
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ch = None
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return int(values[0]), ch
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def contains(source, inst):
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"""
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Returns True if the "source" contains an instance of "inst". False,
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otherwise.
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"""
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if isinstance(source, inst):
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return True
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if isinstance(source, NonCapture):
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for elt in source:
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if contains(elt, inst):
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return True
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return False
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def flatten_result(source):
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"""
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Turns the given source sequence into a list of reg-exp possibilities and
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their arguments. Returns a list of strings and a list of argument lists.
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Each of the two lists will be of the same length.
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"""
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if source is None:
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return [''], [[]]
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if isinstance(source, Group):
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if source[1] is None:
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params = []
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else:
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params = [source[1]]
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return [source[0]], [params]
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result = ['']
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result_args = [[]]
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pos = last = 0
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for pos, elt in enumerate(source):
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if isinstance(elt, basestring):
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continue
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piece = ''.join(source[last:pos])
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if isinstance(elt, Group):
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piece += elt[0]
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param = elt[1]
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else:
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param = None
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last = pos + 1
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for i in range(len(result)):
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result[i] += piece
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if param:
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result_args[i].append(param)
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if isinstance(elt, (Choice, NonCapture)):
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if isinstance(elt, NonCapture):
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elt = [elt]
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inner_result, inner_args = [], []
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for item in elt:
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res, args = flatten_result(item)
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inner_result.extend(res)
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inner_args.extend(args)
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new_result = []
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new_args = []
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for item, args in zip(result, result_args):
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for i_item, i_args in zip(inner_result, inner_args):
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new_result.append(item + i_item)
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new_args.append(args[:] + i_args)
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result = new_result
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result_args = new_args
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if pos >= last:
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piece = ''.join(source[last:])
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for i in range(len(result)):
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result[i] += piece
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return result, result_args
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